Built-in Wizard of Digitization

You select one or several areas and start the Wizard of Digitization. On steps 1 and 2 you select mapping region, and set the scale by setting X left, X right, Y top and Y bottom values. On steps 3 and 4 graphs are handled automatically. On step 5 you can manually add single data points and remove mistakes. 

Important: Only points inwardly selected areas will be digitized.

On a step 1 you need to select mapping region.

On a step 2 set new ranges of X and Y in mapping region you selected.

On a step 3 select parameters to digitize data automatically.

On a step 4 select pixel colors to digitize data automatically.

First of all you need to add colors in list. To do it, move mouse cursor on the picture, select data point you want to digitize, and push key <C>.  Then select colors 'checked' and click <Digitize> button. 

On a step 5 you can remove mistakes, and add single data points manually.

Move mouse cursor on the digitized point. To remove point push key <Delete>.  To add point push key <Insert>. 

On a step 6 data are prepared, a new series of points will be built.

Trick: Repeat steps (4-5-6), on step 4 select next color,  and use button <Add Series> to digitize the graph with new color, and to add next series of points.

 

See example, Tool for digitizing data from an image of graph or plot.