Secondary x-axis numberline doesn't show on histogram plots

[marc1uk]

I'm trying to make a plot that histograms two variables (min, max) for two sources (two different LEDs). The plot should have two y axes and two x-axes; both sets of minimum values are plotted with a given pair of x-y axes, and both sets of maximum values on the other. This appears to be straightforward to implement; just specify the two additional axes in the layout and set the corresponding traces to use them, but for some reason although the secondary x-axis title is shown at the top (as desired), the corresponding axis numberline isn't. reproducible example Apologies for the somewhat contrived example code, it's hacked out of a wider webpage, but i think is easy enough to understand.