First thing to do is to check the "mode" of your histogram. Darktable has three ways to visualise the histogram. One is the 'normal' one, which is linear in intensity, the second is logarithmic in intensity, and the third is a "waveform" display (I never figured out what the use of that one is). To see which mode is displayed, hover your mouse over the histogram. Four squares will appear: left to right display mode, red channel, green channel, and blue channel. Clicking on those squares cycles through the states.

So try clicking (perhaps a few times) on the left square until it shows a straight line (log mode will show a curve, waveform mode some kind of squiggles).

Then, there are a few basecurves for Canon provided. Perhaps one of the alternatives gives a better results? Nothing obliges you to use the camera basecurve. The manual even suggests not using that module at all, if you want to take full control through the "tonecurve" or "zone system" modules.

The basecurves are supplied as an easy way to get close to your camera's rendering of the raw files. Some, however, rise very fast, and can easily lead to clipping in the final image (on which the histogram is based). Try using the "cubic spline" to see the image without any curve applied.