Last I read (somewhere on meta.stackexchange), there's some minimum number of pixels that have to be available on your display in order for the cumulative rep chart to be shown. I guess this makes sense to the UX designers, but for me the effect was that sometimes I'd see it and sometimes I wouldn't. Frustrating.
Anyways, my recommendation is to be on the not-mobile version, go to your math.stackexchange user page, pick your 'Activity' tab, and make your browser window as wide as possible. You should be able to have the option to view it then.
Here are my results, at two slightly different browser widths:
Narrower:
Wider:
It even updates dynamically - have fun watching it blink in and out of existence as you resize your browser window!