There appears to be an issue with WebGL detection on larger iPad Pro 12.9" tablets. I found ticket #280 that addressed this at one point for other iOS devices, but not sure if this is a regression or not. Additional details can be found here of the problem. It seems that if the browser is made smaller the graphing works fine, but once maximized to the full size of the tablet, the error "WebGL is not supported by your browser" is displayed. The same result happens for this user as well on the example pages here.
There appears to be an issue with WebGL detection on larger iPad Pro 12.9" tablets. I found ticket #280 that addressed this at one point for other iOS devices, but not sure if this is a regression or not. Additional details can be found here of the problem. It seems that if the browser is made smaller the graphing works fine, but once maximized to the full size of the tablet, the error "WebGL is not supported by your browser" is displayed. The same result happens for this user as well on the example pages here.