I need to mark certain values in a plot without changing axes limits, e.g., I want a vertical line at x=pi/2 to mark the max(cos(x)) but I don’t want to figure out the appropriate y limits and I just want my vertical line to go from 33% to 67% of the axes. So, as per vertical & horizontal lines in matplotlib, I am using axvline.

Now, I want to annotate the vertical line with vertical text next to it (thus annotate is out).

Thus I need to use text (see, e.g., Add text next to vertical line).

Alas, text requires x and y in data coordinates. Yes, “the coordinate system can be changed using the transform parameter“, but I need x in data and y in axis coordinates!

So, how do I annotate an axvline with vertical text?