Property zerolinelayer for cartesian axes

[my-tien]

Adds a new property zerolinelayer to cartesian axes to allow drawing the zeroline above traces.

This addresses the issue that sometimes the zeroline can be barely visible:

When setting the new property yaxis.zerolinelayer = "above traces":

Disclaimer

I am required to add that…

the software is provided "as is", without warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose and noninfringement. in no event shall the authors or copyright holders be liable for any claim, damages or other liability, whether in an action of contract, tort or otherwise, arising from, out of or in connection with the software or the use or other dealings in the software.

[my-tien]

@gvwilson Do you have a means to trigger the CI again? It seems that this run had a connection error on launch.

[gvwilson]

@my-tien I just re-ran CI - hope that helps.