The units for tickangle is degrees when using scatter:

Plotly.plot(Tabs.fresh(), [{x:[1,2,3], y:[2,1,2], type:'scatter'}], {yaxis:{tickangle:-10}});

However, it switches to radians when using scattergl:

Plotly.plot(Tabs.fresh(), [{x:[1,2,3], y:[2,1,2], type:'scattergl'}], {yaxis:{tickangle:-10}});

This could be a bug in the underlying gl-plot2d library, it doesn't seem to specify what units are expected here.

Also, there is a test gl2d_axes_labels that does specify tickangle: 45, but it turns out that 45 radians looks visually similar to 45 degrees by coincidence, approximately 58 degrees to be more precise. It does appear that this test's baseline "correct" image does have a 58 degree slant baked into it.