I particularly like the automatic mode. However, the deafult 'distance' threshold of 120 seems to be too large: selecting the yellow foreground color picks points on both the yellow and red lines; selecting the green foreground color picks points on both the green curve and the axes.
A solution for the test image is to just reduce the threshold to 50.
The best threshold may depend on both the target color and the specific image being processed, but could potentially be automatically chosen: imagine constructing a greyscale image by replacing each pixel's color with its distance from the target color, and then picking a threshold for converting this grayscale image to a binary image (e.g. using Otsu's method [0]).
The export to plot.ly feature is also great - you can really quickly extract data from one graph and re-plot it in a different/clearer way.
A clustering method like k-means might work, but I've had a quick play in Matlab and the results weren't great.
There are a lot of things that I want to add to this app in the future - e.g. working with heat maps, path and area measurement, simple image editing tools etc. Also while the auto extraction stuff works ok at the moment, I think there is still a lot of room for improvement. I hope I can get to all these things soon.
If you have any suggestions, then feel free to comment here, or use the issue tracker on GitHub. You can also just send me an email.
One minor thing, it could make sense to decrease the default colour distance in the example, as with the current default setting the orange and red lines are selected together in automatic mode when using the orange from the "dominant colors". A distance of 78 or less separates them.
