daedbe

Actually, it looks like Excalidraw[1] to me. The shading and line style match very closely.

[1] https://excalidraw.com/

I often see charts produced using matplotlib or plotly - often you can tell based on the colour schemes used. For example, the bar chart at the bottom of this paper looks like it was made with plotly. I think the reason for such variance in the style of charts is largely due to the flexibility frameworks such as matplotlib provide: you can control basically every aspect of a chart and use any number of predefined or custom stylesheets to change the look and feel.

A common alternative solution in this case would be to use an adaptive thresholding technique such as Otsu’s method.