I set the number format on my QCustomPlot instances to use the so called beautiful powers mode for the tick labels, but I've never managed to actually make it work. I did a little investigation and it turns out the problem is that the various bits of code that parse the label text search for 'e' as the exponential character. The exponential character is locale dependent, in my case (en_GB) it's 'E'. It would probably be better to use QLocale::system().exponential() instead of statically using 'e'.