Hacking at 0300

A simple walkthrough to use the Engauge Digitizer to pull values off a "printed" graph

There's a far more complete manual that comes with the download, but these are the steps I used to generate the data for the webservices graphs. It assumes you've got a black and white image of the graph, with continuous lines for the data and orthogonal gridlines, and linear axes.

We'll use the bilirubin risk levels from the AAP bilirubin guidelines:

1. Open the digitizer

   You can't use Open with... from the context menu because the program expects filenames as part of the options; you can read the manual to see how to modify the context menu. Click Import on the toolbar: and select your image file:

   

   Save your work (it's a .dig file) often. You don't need me to remind you.

2. Set the axes

   Click Axis Point in the toolbar: and click on the origin (lower left corner) of the graph and enter the x,y values for the origin in the units of the graph (not pixels!). Usually that's (0,0) but can be anything. Click on the maximum of the y-axis (upper left corner), then the maximum of the x-axis (lower right corner).

3. Remove the grid

   Select Grid Removal... from the Settings menu. Select "Remove thin lines parallel to the axes" and play with the "Minimum line thickness" until the gridlines are gone from the preview on the bottom but the real curves are still there.

   

   Now the grid is gone and digitizing the curves will be easy:

   

4. Create curves

   Select Curves... from the Settings menu. Create as many curves as you need (we have three). Give them descriptive names if you like:

   

5. Digitize each curve

   Click Segment Fill in the toolbar: . For each curve, select it from the curve menu: and click the points on the graph for that curve until it is covered with points. There's no undo command. If a wrong point is included, use the Select tool: to select the bad points and use the Delete key. To add individual points, use the Curve Point tool: . In a few clicks, you have the fully digitized graph:

   

6. Set up the export

   If you exported the data now, you'd have lots of points; probably far more than is justified from the original graph. Decide which x-axis values should be exported (the program will interpolate appropriately if the digitized points don't land exactly). Select Grid Display... from the Settings menu:

   

   The values to be specified are related by count = (stop - start)/step so you only need to specify three; select the one you don't want to specify from the Disable dropdown menu. Make sure that the start and stop values are actually in the curve; the program will happily extrapolate off the curve but it's probably not scientifically legitimate. Ignore the y axis grid lines.

   Select Export Setup... from the Settings menu. In the Points Selection section, select Intepolate Y's at gridline X's.

   

7. Export

   Click the Export icon in the toolbar: . Pick a name and you now have a CSV file with your data. Simple, no?