Use ax.axvline(x=...) for a vertical reference line that spans all or part of an Axes. If the line should run only between two y values in your data, use ax.vlines(...) instead.
Draw a full-height vertical reference line
Call ax.axvline() on the Axes where the line belongs. The x position is in data coordinates, and the line spans the Axes vertically by default.
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import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([0, 1, 2], [2, 1, 3])
ax.axvline(x=1, color="red", linestyle="--", linewidth=1.5)
plt.show()This adds a dashed red marker at x=1. The official Matplotlib axvline API documents the method and its styling properties. Although plt.axvline(...) is also available, ax.axvline(...) makes the target Axes explicit.
Set the line’s height as a fraction of the Axes
The ymin and ymax arguments to ax.axvline() are Axes fractions, not y data values: 0 is the bottom and 1 is the top. For example, this draws the line through the middle half of the Axes, regardless of the plotted y limits:
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ax.axvline(x=1, ymin=0.25, ymax=0.75)Draw a segment between y data values
Use ax.vlines() when the endpoints should be specific y values in the data coordinate system. Unlike axvline(), its ymin and ymax arguments are y data values.
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([0, 1, 2], [2, 5, 3])
ax.vlines(x=1, ymin=2, ymax=5, color="black", linestyle=":")
plt.show()Here the dotted segment at x=1 runs from y=2 to y=5. vlines() also accepts array-like x positions and endpoints, so it can draw multiple vertical lines. Its API and return type are described in the Matplotlib Axes.vlines reference.
Style or move the line
Pass ordinary line properties such as color, linestyle, and linewidth to axvline() or vlines(). For instance, color="grey" creates a subdued reference marker; the Matplotlib infinite-lines gallery shows a styled reference-line example.
axvline() returns a Line2D. To move a line after creating it, keep that object and set both x endpoints to the new x value:
line = ax.axvline(x=1)
line.set_xdata([3, 3])The endpoints share the same x coordinate, which keeps the line vertical.
Quick Recap
Which method should you use?
- Use
ax.axvline(x=...)for a full-height reference marker. - Use
ax.axvline(x=..., ymin=..., ymax=...)when the vertical extent is a fraction of the Axes height. - Use
ax.vlines(x=..., ymin=..., ymax=...)when the endpoints are y values in your data.
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