How to Create a Data Set From a Hard-Copy Graph

Graphs can help us visualize data, letting us see patterns in information that are difficult to detect in columns of numbers. But sometimes it is necessary to work the other way around--to extract columns of data from a graph. There are times when looking at the actual data set depicted by a graph can give you additional insight into the relationships between the numbers. Preparing a data set can also let you know when a graph is misleading or improperly designed. Extracting data from a hard-copy graph takes patience and a careful eye, but it's really not difficult to do.

Instructions

Extracting a Data Table From a Bar Graph

1

Create two columns on a piece of paper. The first column should have the same name as the x- (or horizontal) axis on the bar graph, and the second column should have the same name as the y- (or vertical) axis. For instance, if you're working with a chart that shows average temperature by month, your first column would be labeled "Month" and the second column would be labeled "Average Temperature."

2

Write the x-axis value for the first bar in the chart under the first column. In our month/temperature graph, the first x-axis value might be "January."

3

Place the ruler so that the top edge is lined up with the top of the first bar, and is parallel to the x-axis and at a 90-degree angle to the y-axis.

4

Note where the ruler crosses the y-axis and write down the value in the second column. If the place where the ruler crosses is between two y-axis labels, estimate the value based on the distance between the two labels. For instance, if the ruler crosses the y-axis midway between "30" and "40," you would write down "35" in the second column.

5

Continue this procedure for the next bar in the graph. Continue until your table has an x-axis value and a y-axis value for every bar in the graph.

Extracting a Data Table From a Line Graph

6

Prepare your table with two columns. The first column should have the same label as the x-axis and the second column should have the name of the y-axis. For instance, in a graph showing how profits change with investments, you would label the first column "Investments" and the second column "Profits."

7

Find the first value marking on the x-axis and write down this value in the first column of your table. In an investments/profits graph, the investment axis might be marked off in thousands of dollars, so the first x-axis value you would write down would be "$1,000."

8

Line up the 90-degree angle of a drafting triangle with the x-axis value in step 2 so that the horizontal side of the triangle lines up with the x-axis value and the horizontal line of the triangle lines up with the rest of the x-axis.

9

Find the spot where the horizontal line of the triangle touches the data line and make a small pencil mark there.

10

Turn the triangle around and line up the horizontal side with the pencil mark you made on the data line; the vertical side of the triangle should line up with the y-axis.

11

Note where the horizontal side of the triangle is crossing the y-axis and write down this value in the second column. If it touches the y-axis between two value markers, estimate the value based on the distance between the two markers. For instance, if the horizontal side crosses the y-axis about three-quarters of the way between "100" and "200," you would write down "175" in the second column.

12

Continue this process for each of the value markers on the x-axis until you have an x-axis value and a y-axis value for every value marking on the x-axis.