xmin and xmax are in Axes units, not data units; using a datetime there would not work. Are you sure you didn't mean ax.axhline(dt)?

The same error would occur with ax.axvline(0, dt), since ymin/ymax have the same usage.

I know that it doesn't work currently -- that is why it's a feature request. It appears very odd that I can easily specify my vertical line on a data coordinate, but if I want to specify the start and end points of a horizontal line, I can no longer use data coordinates.

There is no difference between axvline and axhline; you are calling them differently and passing your datetime to a different argument that does a different thing.

axhline and axvline are meant to be in mixed coordinate spaces, one data space and one Axes space. If you are trying to use data coordinates for both, then you should use plot.

To use axhline effectively with dates on the x-axis, you'll need to simulate a horizontal line across a range of dates by combining axhline with ax.set_xlim( ). Since axhline doesn't directly support date ranges for xmin and xmax, you use axhline to draw the line at a specific y-value and then adjust the x-axis limits to ensure that the horizontal line spans the desired range of dates.

Here’s how to do it:

• Draw the Horizontal Line: Use axhline to add the line at a specific y-value.
• Adjust X-Axis Limits: Ensure that the x-axis limits include the range of dates you want to span.

As mentioned above, axhline x limits are in Axes units (span the full or a fraction of the axis). It's not intended for data units. Use plot([xmin, xmax], [y, y]) or hlines(y, xmin, xmax) if you want x in data units.

Action: The documentation should be improved axhline reads "Add a horizontal line across the Axes", which is technically correct, but it's not obvious that it's not for data coordinates. We should make this more clear.

As mentioned above, axhline x limits are in Axes units (span the full or a fraction of the axis). It's not intended for data units. Use plot([xmin, xmax], [y, y]) or hlines(y, xmin, xmax) if you want x in data units.

Action: The documentation should be improved axhline reads "Add a horizontal line across the Axes", which is technically correct, but it's not obvious that it's not for data coordinates. We should make this more clear.

You're correct that axhline is designed to work with axes units rather than data units, which can be a bit confusing when you want to place a horizontal line based on specific data points on the x-axis.

Summary

• axhline: Best for when you want a horizontal line across a fraction of the axis, not specific data values.

• plot([xmin, xmax], [y, y]) and hlines(y, xmin, xmax): These should be used when you want your line's start and end points to correspond to specific x-axis data values.