Converting Graphs to Tables of Data

Importing text data into Excel for analysis is pretty straightforward, but what if you want to use data from a chart in a book or some other reference? There’s a free tool that will let you convert graphical charts into text data: Engauge Digitizer.

Go to https://markummitchell.github.io/engauge-digitizer/, and click “Latest Releases.” Choose the appropriate installer for your operating system.

The file stress_concentration_hole.jpg is an image of a stress concentration curve for an axially loaded rectangular bar with a hole through the middle. The stress concentration factor is given as a function of the radius of the hole divided by the width of the bar.

To be able to use the data in a spreadsheet, you’ll need to convert the chart to pairs of XY data using Engauge Digitizer.

Once you have the software installed and open, select File > Import, then open stress_concentration_hole.jpg. In the wizard that opens, click Next twice, then Finish.

Any image that’s color or grayscale will look terrible at first, but we can correct it by going to Settings > Color Filter. Adjust the filters by clicking and dragging the green arrows until a recognizable curve appears, then click OK:

Next, you’ll need to tell the software where the axis points are. Select the button with the three red crosses (). Place the crosshair over the intersection of the x- and y-axes. Left-click, then enter the coordinates for this axis point (x=0, y=2).

Repeat for the highest marked point on the y-axis (x=0, y=3.2) and on the x-axis (x=0.5, y=2).

Next, add points by selecting the Point Match tool () and clicking various points on the curve. These are the points that will be exported later as XY coordinates.

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With enough data points selected, the coordinates can be exported to a .csv (comma separated values) file by choosing File and Export. Accept the default name and click Save.

Now Excel can import the data into a worksheet using a text data import. Go to Data > From Text, then double-click on the .csv file that was just created. Make sure the delimited option is selected and click Next. Select the comma delimited option, then click Finish. Choose a location for the data.

To verify that the data was imported properly, select the X and Y column, then create a scatter chart. You can use the Quick Access Toolbar button or go to the Insert tab and find the Scatter Chart button.

Right-click the y-axis and choose Format Axis. Change the minimum and maximum to match the chart in the image (minimum = 2.0, maximum = 3.2). Now the Excel chart is the same as the original:

These data can be used in calculations going forward by using lookups or linear interpolation.

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