When a central bank changes the money supply, what happens to interest rates? That question appears in every introductory economics course, and the answer is usually drawn on one chart. The money market graph is that chart. It shows the supply of money, the demand for money, and the interest rate where the two meet.
The trouble is that “money market graph” can mean two very different things. For a student, it means a textbook diagram with interest rates on the vertical axis. For a trader or analyst, it means a line chart of overnight borrowing rates like SOFR or Treasury bill yields over time. Both are useful. Both tell stories about liquidity. But mixing them up leads to confusion, and a lot of people do exactly that.
This article walks through both versions. It covers how the model works, where it came from, why the textbook version is a simplification, and how to avoid the mistakes that trip up most readers.
Two Charts, One Name
The first version is the money market model. It plots the quantity of money on the horizontal axis and the nominal interest rate on the vertical axis. Two curves appear: a money supply curve and a money demand curve. Where they cross, you get the equilibrium interest rate.
The second version is a money market rates chart. It plots a rate over time. You might see the effective federal funds rate, the Secured Overnight Financing Rate, or the yield on a three-month Treasury bill. There are no supply and demand curves here, just history.
| Feature | Money Market Model | Money Market Rates Chart |
|---|---|---|
| Horizontal axis | Quantity of money | Time |
| Vertical axis | Nominal interest rate | Rate in percent |
| Curves shown | Money supply and money demand | One or more rate series |
| Main use | Teaching how rates form | Tracking actual borrowing costs |
| Typical source | Economics textbooks | Central bank and Treasury data |
Knowing which chart a writer means is the first step to reading it correctly. A headline about “the money market graph turning vertical” almost certainly refers to the model. A headline about “the money market graph spiking” almost certainly refers to rates.
Reading the Supply Curve
In the standard textbook drawing, the money supply curve is a vertical line. The logic is simple: the central bank decides how much money exists, so the quantity does not respond to the interest rate. Whatever the rate, the quantity stays fixed.
That vertical line is a teaching device. It represents a regime where the central bank targets a quantity of money and lets the rate adjust. Most modern central banks do not operate that way. They set a policy rate and supply whatever reserves the banking system demands at that rate.
This matters for anyone building a graph. If you draw a vertical supply curve while describing a modern ample-reserves system, your diagram will contradict your text. The two regimes need different pictures.
Reading the Demand Curve
The money demand curve slopes downward. As the interest rate rises, holding money becomes more expensive because you give up interest you could have earned elsewhere. Economists call that forgone interest the cost of holding money. When that cost goes up, people hold less cash and more interest-bearing assets.
This downward slope reflects real behavior; it is more than a textbook convenience. Households keep less in checking accounts when savings rates are attractive. Firms manage cash more tightly when short-term yields are high. The relationship is one of the most durable in macroeconomics, even though its exact size is debated.
The framework itself traces back to John Maynard Keynes, whose 1936 book The General Theory of Employment, Interest and Money introduced what became known as liquidity preference. Keynes argued that people hold money for transactions, precaution and speculation, and that the interest rate is the reward for parting with liquidity.
Where the Curves Meet
The intersection of supply and demand gives the equilibrium interest rate. This is the rate at which the amount of money people want to hold equals the amount that exists.
The comparative statics are what most exams test. If the money supply shifts right, the equilibrium rate falls. If money demand shifts right, the equilibrium rate rises. These shifts explain why central banks easing policy tends to push short-term rates down, and why strong demand for cash can push rates up even without any policy change.
A practical tip: when you draw this by hand or in a spreadsheet, label the axes before anything else. Most errors come from swapping the axes or forgetting that the interest rate sits on the vertical. Once the axes are right, the rest follows.
Why the Textbook Version Is a Simplification
Here is where the graph gets interesting, and where a lot of commentary goes wrong.
Since the 2008 financial crisis, and again since 2019 and 2020, the Federal Reserve has managed the federal funds rate mainly through administered rates. It pays interest on reserve balances and offers an overnight reverse repurchase facility. These set a floor and a ceiling, and the market rate trades inside that corridor. In an ample-reserves regime, the supply of reserves is effectively horizontal over a wide range at the administered rate, not vertical. (Federal Open Market Committee)
So the diagram changes shape depending on the regime:
- Scarce reserves, quantity target: vertical supply curve, rate adjusts.
- Ample reserves, rate target: horizontal supply curve at the policy rate, quantity adjusts.
Neither drawing is wrong. They describe different operating systems. The mistake is presenting one as if it were the other. If a chart claims to show how the Fed sets rates today but draws a vertical supply curve, the author is teaching a model that no longer matches the plumbing.
The cost of misunderstanding this plumbing is real. Institutions spend heavily on systems and staff that track liquidity and rate dynamics, and the spending breakdown is worth understanding in its own right, as this analysis of where identity threat detection money actually goes shows for a parallel case.
The Rates Version: What the Lines Actually Are
Now to the second chart. When analysts plot money market rates, they usually pick from a small set of benchmarks.
The effective federal funds rate is the volume-weighted median of overnight unsecured borrowing between banks. It is published daily and is the closest thing to “the” U.S. policy rate in practice.
The Secured Overnight Financing Rate, or SOFR, is based on overnight repurchase agreements backed by Treasury securities. It replaced LIBOR as the main dollar benchmark after LIBOR’s panel publication ended in mid-2023. SOFR is secured, so it behaves differently from unsecured rates during stress.
Treasury bill yields at three and six months are the classic market-based reference. They reflect what the government pays to borrow short term and move with expectations about policy.
A good rates chart plots at least two of these together. The spread between them tells you about collateral scarcity, credit concerns and market functioning. A single line tells you almost nothing on its own.
Common Mistakes People Make
Several errors show up again and again, both in classrooms and in published commentary.
Mixing the two chart types. A model diagram has no time axis. A rates chart has no demand curve. Combining them produces nonsense.
Forgetting the regime. Drawing a vertical supply curve while discussing a rate-targeting central bank is the single most common conceptual error.
Ignoring the axis labels. Interest rate on the vertical, quantity on the horizontal. Swapping them inverts every conclusion.
Treating money as one thing. Cash, checking deposits, reserves and money market fund shares are not interchangeable. The definition used, M1 or M2, changes the shape of the demand curve.
Reading a single day as a trend. Money market rates are noisy. One spike in SOFR around quarter-end is usually a balance-sheet effect, not a policy shift.
Skipping the source notes. Rate series get revised. Methodologies change. Always check the publication date and the definition before drawing conclusions.
There is a further trap for anyone who reads financial news casually. Stories about scams and lost funds often blur into stories about market structure, and readers carry the confusion forward. A clear example is how the gift card scam works and why victims rarely get their money back, which has nothing to do with interest rates but everything to do with how money moves between parties.
Building a Money Market Graph That Holds Up
Whether you are studying or presenting, a few habits separate a clean chart from a misleading one.
- Decide which chart you are making and say so in the title.
- Label both axes with units.
- State the reserve regime if you are drawing the model.
- Use official data series for rates, not screenshots.
- Plot a spread when comparing benchmarks, rather than a single line.
- Note the date range and any methodology changes.
For the model, the cleanest approach is to draw the standard version first, then add a second panel showing the ample-reserves version. Side by side, the two panels teach more than either alone. Readers see immediately why the shape of the supply curve depends on how the central bank operates.
For rates, a two-panel layout also works well. Put the policy rate on top and the benchmark spreads below. The eye picks up divergences fast.
Where the Money Market Fits in the Wider System
The money market is where short-term liquidity gets priced. That makes its graph a window into the whole financial system. When the curves behave oddly, something upstream is usually moving.
Central bank digital currency is one such upstream force. A retail CBDC would change the composition of money and could shift the demand curve in ways the standard model does not capture. The debate over how a CBDC would work as central bank money going digital is worth following for anyone who tracks these diagrams.
New technology is another. Automated trading and AI-driven cash management change how fast money moves between instruments, which affects the slope of demand. The question of how to make money with AI is mostly a business question, but the underlying shift in cash management is a money market question.
Prediction markets have also started pricing policy and political outcomes that feed into rate expectations. A contract on something as distant from rates as a California independence vote in 2026 prediction market still tells you something about how traders price tail risks, and tail risks are what money market graphs tend to show during crises.
Capital flows matter too. The comparison of Rocket Lab versus SpaceX and their very different market caps in 2026 is a reminder that valuations swing on expectations, and expectations about growth feed into the demand for liquidity. Similarly, the surge in prediction markets around open source frontier AI in 2026 shows how quickly new instruments can absorb attention and capital that once sat in cash.
None of these forces replaces the basic model. They change the inputs. A reader who understands the underlying graph can interpret the headlines without getting lost.
Conclusion
The money market graph is two tools wearing one name. The model shows how supply and demand set the interest rate. The rates chart shows what that rate has actually done. Both reward careful reading, and both punish sloppy labels.
The single most useful habit is to ask which regime a chart depicts before trusting its shape. A vertical supply curve belongs to a quantity-targeting world. A horizontal one belongs to a rate-targeting world. Modern central banks live mostly in the second, which is why the textbook drawing needs a companion panel rather than a replacement.
Get the axes right, name the series, and state the regime. Do that, and the graph stops being a confusing diagram and starts being what it was always meant to be: a clear picture of how the price of money gets set.
What does a money market graph show?
It shows the relationship between the quantity of money and the nominal interest rate. The supply curve reflects how much money exists, and the demand curve reflects how much people want to hold at each rate. Their intersection is the equilibrium interest rate. A separate version of the graph plots actual rates like SOFR or Treasury bill yields over time.
Why is the money supply curve vertical in textbooks?
Because the textbook model assumes the central bank fixes the quantity of money, so the amount does not change when the interest rate moves. That assumption fits a scarce-reserves, quantity-targeting system. In a modern ample-reserves system, the supply of reserves is better drawn as horizontal at the administered policy rate.
Why does money demand slope downward?
Holding money means giving up interest you could earn elsewhere. As rates rise, the cost of holding money rises, so people hold less money and more interest-bearing assets. The result is a downward-sloping demand curve, a relationship central to Keynesian liquidity preference.
What is the difference between the model and a rates chart?
The model has quantity on the horizontal axis and the interest rate on the vertical, with two curves. A rates chart has time on the horizontal axis and a rate series on the vertical. One explains how rates form. The other records what they did. They are not interchangeable and should never be combined in a single drawing.
Which rates appear most often on money market charts?
The most common are the effective federal funds rate, SOFR, and three- and six-month Treasury bill yields. SOFR replaced LIBOR as the main dollar benchmark after LIBOR panel publication ended in mid-2023. Plotting a spread between two of these series usually tells you more than plotting one alone.
How do I know which reserve regime a chart assumes?
Look at the shape of the supply curve. A vertical line implies a quantity target with scarce reserves. A horizontal line at the policy rate implies ample reserves and rate targeting. If the chart does not make the regime clear, treat its conclusions with caution until you can confirm which system it describes.
