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Use ax.axhline(y=..., linestyle="--") to draw a dashed horizontal line at a y value across a Matplotlib axes. For example:
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([0, 1, 2], [1, 3, 2])
ax.axhline(y=2, color="gray", linestyle="--", linewidth=1.5)
plt.show()The line marks y=2 on the plot. Matplotlib accepts "--" and "dashed" as equivalent ways to request the standard dashed style, as shown in its plot reference.
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Choose between axhline and hlines
Pick the function according to how you want to specify the line’s horizontal extent:
| Function | Horizontal extent | Best used when |
|---|---|---|
ax.axhline(y=...) |
xmin and xmax are axes coordinates from 0 to 1. |
The line should cross the axes, or its ends should be placed as fractions of the axes width. |
ax.hlines(y, xmin, xmax, ...) |
The x endpoints are data coordinates. | You need to set the line’s start and end using x values on the plot. |
With axhline, the y position is a data coordinate, while xmin and xmax are fractions of the axes width—not x values from your dataset. For example, to draw only across the middle half of the axes:
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ax.axhline(y=2, xmin=0.25, xmax=0.75, linestyle="--")Use hlines when you want the endpoints expressed as data values; Matplotlib’s axhline reference points to that function for data-coordinate x limits.
Customize the dash pattern and appearance
For the standard dashed line, set linestyle="--" or linestyle="dashed". You can also adjust properties such as color and width:
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ax.axhline(y=2, color="black", linestyle="dashed", linewidth=1.5)For a custom pattern, give the returned line a sequence of alternating on and off lengths, measured in points:
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line.set_dashes([6, 3]) # 6 points on, 3 points offHere, 6 is the dash length and 3 is the gap length. Matplotlib describes dash sequences and further options in its dash-control example.
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Change the line style after creating it
axhline returns a Line2D object, so you can save it and set its linestyle later:
line = ax.axhline(y=2, color="gray")
line.set_linestyle("--")This is useful when the line is created in one part of your code and styled elsewhere. The returned object also supports the line properties used for color and width.
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