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A Simple Method for Obtaining Original Data from Published Graphs and Plots

Chris L. Sistrom1 and Patricia J. Mergo

1 Both authors: Department of Radiology, University of Florida College of Medicine, P. O. Box 100374, Gainesville, FL 32610-0374.



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  		Fig. 1. —Graph shows scanned plot with scan and plot coordinate systems labeled..TXT file produced by NIH Image will contain numbers in scan coordinate spaces (X and Y pixel values). Spreadsheet operations serve to convert these into plot coordinates (Abs = abscissa, Ord = ordinate, Org = origin, Max = maximum), which are original data values.

 


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  		Fig. 2. —Reproduction of image file produced by scanning original scatterplot used to test our method. Weight of 227 patients was plotted against age in study of prevertebral soft-tissue thickness on cervical spine radiographs.

 


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  		Fig. 3. —Scatterplot produced after importing derived data values for weight and age into graphing program (PowerPoint; Microsoft, Redmond, WA). Note that distribution of data points is visually indistinguishable from original material in Figure 2.

 


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  		Fig. 4. —Graph shows error terms for patient age (in years) derived by conversion of scanned plot (Fig. 2) then compared with original data. Difference between original and derived ages (orig—deriv) is plotted against original age values. Derived age equaled original age for every point.

 


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  		Fig. 5. —Graph shows error terms for subject weight (in pounds) derived by conversion of scanned plot in Figure 2 then compared with original data. Difference between original and derived weights (orig—deriv) is plotted against original weight values. Mean error (solid line) and limits of agreement (±2 SD, dotted line) are shown.

 

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