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Core PHP Programming: Using PHP to Build Dynamic Web Sites by Leon Atkinson

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Bar Graphs

Bar graphs are a good way to compare values to each other. Creating them is a relatively simple task because each data point is a rectangle. The height of the rectangle represents the value of the data point. To make the transition, a scaling factor is used. In Listing 19.3 the graph is 200 pixels tall and the scaling factor is two. This means that a data point with the value 75 will be 150 pixels tall.

The business of creating the graph is similar to the process described above where a button is created. A blank image is created, several colors are allocated, and functions are called for drawing shapes into the image. The script allows the width of the bars to adapt to the width of the graph. The width of the graph is divided by the ...

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