Adding markers at constant X-value on skew-T plot

[DanielAdriaansen]

Greetings. I am trying to add markers at a constant x value at each pressure level in a skew-T plot, such that they are vertical and not skewed, colored and symbolized in various ways to signify criteria or conditions at each level in the sounding. I tried using this MetPy Monday example:
https://www.youtube.com/watch?v=yrgVrUEm7x4

To try and figure out what flavor of transformation I needed for my x-coordinate to pin the markers on the y-axis. Here is some code that I tried. Only one line produces values, which occur along the skewed Temperature line and not on the y-axis where I'd like them.

from metpy.plots import SkewT
import matplotlib.pyplot as plt
import numpy as np
import xarray as xr

prs = np.linspace(1000.0,400.0,16)
tmp = np.linspace(5,-10,16)
dew = tmp-5

fig1 = plt.figure(1,figsize=(30,20))
skew1 = SkewT(fig1,rotation=45.0)

skew1.plot(prs,tmp,'r')
skew1.plot(prs,dew,'g')

xval = -20

skew1.plot([xval]*len(prs),prs,marker='*',markersize=50,color='orange')
skew1.ax.plot([xval]*len(prs),prs,marker='*',markersize=50,color='orange') # This line produces the markers
skew1.ax.plot([xval]*len(prs),prs,marker='*',markersize=50,color='orange',transform=skew1.ax.get_yaxis_transform(which='tick2'))
skew1.ax.plot([xval]*len(prs),prs,marker='*',markersize=50,color='orange',transform=skew1.ax.get_yaxis_transform(which='tick1'))
skew1.ax.plot([xval]*len(prs),prs,marker='*',markersize=50,color='orange',transform=skew1.ax.get_yaxis_transform(which='grid'))

#skew1.scatter([xval]*len(prs),prs,marker='*',s=50,color='orange') # Does not work
skew1.ax.scatter([xval]*len(prs),prs,marker='*',s=50,color='orange')
skew1.ax.scatter([xval]*len(prs),prs,marker='*',s=50,color='orange',transform=skew1.ax.get_yaxis_transform(which='tick2'))
skew1.ax.scatter([xval]*len(prs),prs,marker='*',s=50,color='orange',transform=skew1.ax.get_yaxis_transform(which='tick1'))
skew1.ax.scatter([xval]*len(prs),prs,marker='*',s=50,color='orange',transform=skew1.ax.get_yaxis_transform(which='grid'))
plt.show()

Any tips on how to transform my x-coordinate from the skewed to vertical values, or how to reference the underlying axis/axes properly? Thank you!

[DanielAdriaansen]

I was able to get it to work by changing my x-value to "axes coordinates" (see here: https://matplotlib.org/stable/users/explain/artists/transforms_tutorial.html#axes-coordinates).

The selection of the x-value is somewhat arbitrary but is fairly easy to approximate where you'd want it to appear.

One problem is if I selected xval=0.0, the marker was hidden behind the y-axis rather than plotted over it, even after tinkering with zorder. But it looks just fine at xval=0.05 or greater.

xval = 0.05
skew1.ax.plot([xval]*len(prs),prs,marker='*',markersize=50,color='orange',transform=skew1.ax.get_yaxis_transform())