These are obviously plots on paper. If you can rescan them or take new photos in good light, that really would pay off. Poor contrast and non-uniform light makes all quite tiresome. Uniformly lighted high contrast images are easily traced in Inkscape but these will provide garbage. Two workarounds are already shown:
- trace in pieces and combine
- draw it with the pen
I suggest doing the job in a photo editor (Photoshop, GIMP) as a way out having only that one image:
- Make the red graticule as light as possible and desaturate it. Use Hue/Saturation tool selectively. Adjust the saturation to the minimum, find the range selector slider positions which take all of the red and drag the lightness slider to maximum:
- There's still left much remnants of the graticule. Fortunately they everywhere have finer grain and they are lighter than the curve - at least when compared to the nearest part of the curve. Apply Filter > Blur > Smart Blur to smooth fime grains but leaving strong local contrasts intact (two images, one for settings and another for the result):
This can already be useful, but let's make it closer to the ideal (=black curve on white background). Tresholding cannot be applied because the brightness is too different in left and right. It must be balanced, add brightness to the darker areas. This can be done in several ways:
- insert a gradient layer with blending mode = Add.
- the same, but create the gradient by making a heavily blurred and inverted copy, fix the edges manually
- insert an adjustment layer for levels or curves and find the balance with gradient in layer mask
Let's try the first version. Have a gradient from black (left) to 50% grey (right) in a new layer that has blending mode =Add:
That needs a little fine-tuning. Spray to the gradient a little black with large smooth low opacity brush to take back the overwhitened top right corner.The unwhitening is done most easily after the next step:
Add a levels layer to lift light grey to white and pushing the dark grey down to black:
There are still a couple pieces of garbage, but they are far from the curve. Use eraser to fix them.
Finally, if it's wanted, it can be traced in Inkscape. Single treshold BW won't be ok, because there are greyshades => Tracing mode should be greyscale with at least 4 levels.