Gradient

This tool can be used to calculate the angle of a linear gradient. Upload an image and click on the points you wish to sample. The image is also plotted in 3D below, which you can use to verify whether a gradient is in fact linear.

Note that because it’s designed for greyscale images, it only looks at the red channel. It might work for screentones as well if you blur the image beforehand.

Points:
Angle: °