How to Extract Data from Graphs using Plot Digitizer

How to Extract Data from Graphs using Plot Digitizer or Getdata Graph Digitizer

Hossam Zein1,2, Vu Le-Huy Tran2,3, Ahmed Azmy2,4, Abdelrahman Tarek Mohammed2,5, Ali Mahmoud Ahmed2,5, Ahmed Iraqi Ahmed Iraqi2,6, and Nguyen Tien Huy2,7

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Department of Ophthalmology, Faculty of Medicine, Zagazig University, Zagazig City, Egypt

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Online Research Club (http://onlineresearchclub.org)

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University of California, Los Angeles, US

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Faculty of Medicine, Tanta University, Tanta, Egypt

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Faculty of Medicine, Al-Azhar University, Cairo, Egypt

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Benha Faculty of Medicine, Benha, Egypt

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Department of Clinical Product Development, Institute of Tropical Medicine (NEKKEN), Nagasaki University, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan

Data extraction is an essential task in doing research, particularly in systematic review and meta-analysis. Usually the researchers provide their data in their publications or in the corresponding supplemental. However, some articles just present the graphs without any related data. Especially, many articles published long time ago do not have an electronic copy, and they are in a scanned version. In these cases, extracting data from graphs is a big challenge. Some people use a manual approach where they measure the length of the bars in the graph and use the ratio to estimate the actual datum. However, this method is time – consuming when we handle a large amount of data. In this guideline, we describe our experience in the application of two software to extract data from graphs, including Plot Digitizer and Getdata Graph Digitizer. I. Plot Digitizer: Download at http://plotdigitizer.sourceforge.net/ This software requires the java virtual machine to be installed and enabled, which can be downloaded at https://java.com/en/download/ 1. Open an image as shown below:

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The image must be in .jpeg or .bitmap format. Note that this image already has some values and scale. We will use these numbers to evaluate the accuracy of the software.

2. We choose the type of the plot according to whether the x- or y-axes or both are on the log scale or not.

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3. The idea of the program is that we set three calibration points which we are sure about their x and y values. These points must not be on the same line. We open the calibration window as shown in the picture below.

The calibration window:

Then we choose the first point to mark. There is a magnifier window to help us mark the point accurately. We left click on the point we want. Choose the other two points. These three calibration points are denoted as C1, C2, and C3 in the picture below.

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After setting the three points we enter their x and y values

Then we click calibrate. 4. We now begin to mark the points we want to know their values by simply left clicking on it. The x and y values of this point will appear at the left side of the screen. In case we want to remove it, just right click on it 4

In this example, the value given by the graph is -1.37 while the value we get from Plot Digitizer is -1.36919, which is very close. 5. We can also change the names of x and y columns in the sheet to avoid confusion by clicking on the button circled in red in the picture below.

In the Label Columns window, type in the names we want for x and y axes of our figure.

In this example, we labeled X as “time post treatment” and Y as “UI episodes per day.”

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6. We can then export the results to a Microsoft Word file, Excel file, or txt file as shown below.

We can save the image only or save our progress at any time by choosing “Save Project As.” Plot Digitizer also provides more useful functions for users, such as changing the color of the calibration points and point markers, changing the cross hair color and width, and changing the size of magnification. II. Getdata Graph Digitizer: Download at http://www.getdata-graph-digitizer.com/ 1. Open an image as shown below. The following formats are available: TIFF, JPEG, BMP, PCX.

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2. In this software, we set the minimum and maximum values of x and y axes instead of defining the 3 points as described in Plot Digitizer. To start click on the “Set the Scale” button.

We now set up the x minimum value. Left click on the point we want to set and type its value. There is also a magnifier window to help us mark the point accurately

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In order to make the movement of the cursor more precisely on the magnifier window, we can keep the Ctrl key while moving the cursor. Repeating this step to set the x max, y min, and y max values. A dialogue box will appear at the end to confirm our setup. We can change the scale of x or y axis to a log scale and we can change the values we entered.

3. Then we start marking the point we want to know its value. Click on the button shown

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Then left click on the point we want to know its value. Then its x and y values will be shown on the sheet on the right.

We can see that the value is also very close to the value reported in the figure (-1.36823 compared to -1.37). If we make a mistake during the process of marking the point, there is an eraser tool to help us get rid of the previous point.

5. Before exporting the results we want to set the output format. From Settings  Options, the following box appears.

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6. Then we can export the results from File  Export data. The following formats are available:

7. We can save our progress at any time from File  Save Work Space. 10

8- If we mark multiple points, the software doesn't number them automatically on the image as in plot digitizer. However, we have some different tools to help us to reorder the points. For example: we want to mark all the points on the blue and orange lines without getting confused. So we will mark the three points on the blue line using the point capture mode.

From the current status tab we click properties.

We change the name of the line to blue. We can change the color of the line we have just drawn from ‘change color’. To mark the points on the orange line, we choose ‘add line’ from the current status tab and mark the 3 points from the point capture button. This is after we marked them. Note that the sheet on the right displays only the points of the current line.

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we may

Forget which point is number 1 or 2 or 3. Here comes the rule of the reorder button inside the red circle in the above image.

Here we chose the point on the right to be number 0 and we will click on the next one to be number 1. Here is after we finished:

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Points are now reordered from right to left. We can do the same with the blue line after choosing it from the current status tab. Here is how the results appear after export.

Additional options for point colors and other features are explained in the help content of the software. In conclusion, Plot Digitizer and Getdata Graph Digitizer are two simple and convenient tools to extract data from graphs. The accuracy of this method is good, and it can help us save more time when we handle a large amount of work. Plot Digitizer is preferred since it is a free software.

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