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Use ax.axvline(x=...) to add a vertical reference line to a Matplotlib plot. It spans the Axes height by default. If you need a line segment whose top and bottom are specified as y-axis data values, use ax.vlines() instead.
Add a full-height reference line with ax.axvline()
Call ax.axvline() on the Axes that contains your plot. The x argument is the line’s position in data coordinates.
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 draws a dashed red line at x = 1, spanning the Axes height. The Matplotlib API describes axvline as adding “a vertical line spanning the whole or fraction of the Axes.” Matplotlib’s axvline API reference documents the arguments and return value.
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Control how much of the Axes the line spans
For a line covering only part of the plot’s height, pass ymin and ymax as fractions of the Axes height. These values use Axes coordinates, not y-axis data values: 0 is the bottom edge and 1 is the top edge.
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ax.axvline(x=1, ymin=0.25, ymax=0.75)This draws the line through the middle half of the Axes, regardless of the plotted y-data limits. The x position remains in data coordinates.
Use ax.vlines() for endpoints in y data coordinates
When the line should run between actual y values, use ax.vlines(x, ymin, ymax). Unlike axvline, its ymin and ymax arguments are y-axis data values. Matplotlib’s vlines API reference also documents scalar or array-like positions and endpoints.
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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()This example draws a dotted segment at x = 1 from y = 2 to y = 5. You can supply multiple x positions and corresponding endpoints when you need several data-bounded vertical lines.
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ax.axvline() accepts standard line properties, including color, linestyle, and linewidth, as shown in the first example. It returns a Line2D object, which you can keep and update. To move an existing vertical line to x = 3, set both x endpoints to the same value:
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line = ax.axvline(x=1)
line.set_xdata([3, 3])For a pyplot-style call, plt.axvline(...) is also available; Matplotlib documents it as a wrapper for Axes.axvline. Using ax.axvline(...) makes the target Axes explicit. For a runnable reference-line example, see the Matplotlib Infinite lines gallery.
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