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!

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

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.