Slider `marks` keys are only strings

[chriddyp]

we should either enforce that or cast back to integers. See https://community.plot.ly/t/slider-not-working/8471/8?u=chriddyp for context.

[billgreenwald]

It looks like the behavior switched so that keys should be ints/floats, and values should be strings, at some point.

I had the same error, and then swapped my keys and values, and the error went away.

ie i went from

marks={str(float(i)/2): float(i)/2 for i in range(0,19)},

to

marks={float(i)/2: str(float(i)/2) for i in range(0,19)},