I'm using Darktable 2.4.2 with Canon 6D. The problem is that when I import a raw image, and apply the Canon 6D base curve, the histogram is totally different with respect to the histogram visualized on the camera. I attach the image to explain better. I tried to calibrate the base curve using basecurve internal tool with no results. My photos appear too bright. Can you help me?
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.
In the photograph, Darktable's history stack indicates Curva Base [Base Curve] has been applied to the digital file...twice. Usually the first time is a default base curve based on the camera model. Without custom configuration, Darktable applies a tone curve by default to RAW images. Because there are two applications, the Base Curve is definitely not the default, though it may have also been applied automatically. Anyway, any tone curve (gamma adjustment) applied by Darktable, even the default, may differ from the manufacturer's built in tone curve (gamma adjustment) because gamma adjustment is a matter of taste.
It's also worth considering that the single histogram from the camera combines red, green, and blue channels in an arbitrary way to provide information at a glance. With most images, the darkest shadows and brightest highlights will not consist of all channels equally (unless the shadows/highlights are blown to black/white). Interestingly, the Darktable histogram seems to be just for the blue channel. Blue channel extending well to the right is common for daylit images containing sky. Often in such images, the blue channel will also extend well to the left because shadows are illuminated by ambient blue light from the sky.