Plot marker labels in the wrong place when using log scale

[thomashampson]

Describe the bug
Marker labels are drawn in the wrong location when axes are set to log scale.

To Reproduce

1. Open a plot
2. Change x axis to log
3. Add a vertical marker - the label will be drawn in the wrong place
4. Move the marker and the label will move in a funny way

Expected behavior
Behaviour should appear to be the same as with linear scales.