The Tool Desk
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Draw multiple finite horizontal segments with hlines
Axes.hlines() accepts one or more y positions and x-coordinate endpoints. Its xmin and xmax values are data coordinates, so they align with the chart’s x scale.
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([0, 1, 2, 3], [1, 3, 2, 4], marker="o")
ys = [1.5, 2.5, 3.5]
ax.hlines(ys, xmin=0.5, xmax=2.5, colors="tab:red", linestyles="dashed")
ax.set(xlabel="x", ylabel="y")
plt.show()This draws three dashed red segments, all from x = 0.5 to x = 2.5. A scalar endpoint is shared by every line. To give lines different endpoints, provide corresponding array-like values:
ax.hlines([1, 2], [0, 1], [3, 4])The function also accepts per-line colors and line styles when you want to vary their appearance. It returns a LineCollection.
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Draw full-width reference lines with axhline
For thresholds or other reference rules that should cross the plotting area, call Axes.axhline() once for each y value:
for y in (1.5, 2.5, 3.5):
ax.axhline(y=y, color="gray", linestyle="--", linewidth=1)Here, y is a data coordinate. Unlike hlines, however, axhline interprets xmin and xmax as fractions of the axes width: 0 is the left edge and 1 is the right edge. For example, ax.axhline(y=0.5, xmin=0.25, xmax=0.75) draws a line across the middle half of the axes, at y = 0.5 in data coordinates. It returns a Line2D.
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Choose the function by what the line represents
| Use case | Recommended API | Horizontal extent |
|---|---|---|
| Several finite reference segments | ax.hlines() |
x endpoints are data values |
| One or more axes-spanning reference lines | ax.axhline() |
xmin and xmax are fractions of axes width |
| A constant-y trace that is part of the plotted data | ax.plot() |
Supply x and y data as for other plotted series |
When the horizontal traces are actual data series rather than markers or thresholds, use plot. Matplotlib’s plot API supports multiple x/y data sets in one call; shared keyword styling applies to all series unless you provide per-series labels or styles.
Avoid the common coordinate mix-up
Do not pass data values such as xmin=10 and xmax=20 to axhline() when you mean “from x = 10 to x = 20.” Those arguments specify fractions of the axes width, not data-coordinate endpoints. Use ax.hlines(y, xmin=10, xmax=20) for that interval.
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For a collection of similarly styled finite segments, one hlines call can express the shared y positions and extent directly. Use separate axhline calls when individual full-width rules need distinct styling or should be managed as separate Line2D artists.
These examples follow the Matplotlib 3.11.x stable API documentation retrieved on October 4, 2026. Check the documentation for your installed Matplotlib version if you need version-specific behavior.
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Official API references
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