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Use ax.axhline(y=value) for a horizontal reference line that spans the whole Axes or a fraction of it. Use ax.hlines() when the line’s start and end should be specified as x-values in your data.
Draw a horizontal reference line across an Axes
ax.axhline() is the usual choice for a threshold, target, or baseline that should extend across the plot area. The y position is a data value; by default, the line spans the full Axes width.
import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([0, 1, 2], [1, 3, 2])
ax.axhline(y=2, color="red", linestyle="--", linewidth=1.5)
plt.show()In this example, the line marks y = 2. To draw it across only part of the Axes, set xmin and xmax to fractions of its width: 0 is the left edge and 1 is the right edge. For example, ax.axhline(y=2, xmin=0.25, xmax=0.75) spans the middle half of the Axes. These arguments are not x-values from your plotted data. See the Matplotlib axhline API documentation.
Choose between axhline and hlines
The deciding factor is how you want to specify the horizontal endpoints:
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| What you need | Method | Coordinates | Returns |
|---|---|---|---|
| A single reference line across all or part of the Axes | ax.axhline(y=...) |
y is a data value; xmin and xmax are Axes-width fractions |
Line2D |
| A finite segment with data-coordinate x endpoints | ax.hlines(y, xmin, xmax) |
y, xmin, and xmax are data values |
LineCollection |
| Several horizontal lines | ax.hlines(ys, xmins, xmaxs) |
Positions and endpoints can be scalar or array-like | LineCollection |
For example, this draws a segment at y = 2 from data x-value 0.5 to 1.5:
ax.hlines(y=2, xmin=0.5, xmax=1.5, colors="tab:blue", linestyles="dashed")With several levels, pass a sequence for y; scalar endpoints can be shared across the lines. If you also pass arrays for the endpoints, check that their shapes and ordering pair each endpoint with the intended line. The hlines API documentation describes its arguments and return value.
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Style or move the line
axhline accepts line properties including color, linewidth, linestyle, alpha, and zorder. For instance, the first example uses a red dashed line with a width of 1.5.
Because axhline returns a Line2D, you can keep the returned artist and change its y position later by setting both y values:
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line = ax.axhline(y=2)
line.set_ydata([3, 3])hlines returns a LineCollection, not a Line2D; use the collection’s properties when styling or updating lines created with that method. Matplotlib’s Axes API documentation covers Axes artists and methods.
Keep coordinate systems straight
The key distinction is that axhline mixes coordinate systems: its y position is in data coordinates, while its horizontal extent is measured as fractions of the Axes width. With hlines, the y position and both x endpoints are data coordinates.
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Matplotlib’s horizontal and vertical line gallery example also demonstrates using an axis transform when an extent should be tied to the plotted area rather than to data limits. That approach is useful when the line’s vertical coordinate should map to the Axes boundaries: in the illustrated transform, 0 maps to the bottom and 1 to the top.
Quick Recap
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