scattergl calculates wrong error bar value if y value is a string

[slioxo]

I stumbled upon this problem today. As you can see here, when the y-value is given as string and the error value as float, the error value is presumably parsed as string and then added using the "+" operator, which results in string concatenation and therefore a wrong value.

For example, if your y-value is 2 and the error value is 2 then the error bar should be drawn up to 4. When y is a string though, the error bar is drawn up to 22.

When using 'scatter' instead of 'scattergl', this bug does not occur.

[alexcjohnson]

Also it gets autorange even more wrong, and the errorbars disappear on redraw (the latter also reported in #2450)

Thanks for the report!