How to plot a graph or histogram using Opencv?

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I would say: just don't do it, generate the csv files with OpenCV and plot it via python/matplotlib or latex/tikz. You'll get nice plots with much less effort!

The Plot2d module is now available in OpenCv (Contrib). You can see more info about it in this thread where you have an example of how to use it.

You can see how to create/plot a histogram of an image in OpenCV here:

http://docs.opencv.org/doc/tutorials/imgproc/histograms/histogram_calculation/histogram_calculation.html