## Histograms – Remember the GD library?

Published: **December 23rd, 2008** by: **Kurtis**

Better recognized perhaps as "bar graphs" to most people, in the world of statistics, histograms provide a graphic representation of a set of data. This graph can be used to make inferences about the data and draw conclusions about the sample and population. In this article, we'll dive into a few new functions available as part of the GD library that are particularly useful for this application.

<?php $x = explode("a",$_GET['x']); $numClasses = $_GET['nC']; $min = (float)$x[0]; $max = (float)$x[0]; foreach ($x as $val) { if ((float)$val<$min) { $min = $val; } if ((float)$val>$max) { $max = $val; } } $c = ceil((($max-$min)/$numClasses)); for ($z=0; $z<$numClasses; $z++) { $class{$z} = array((($z*$c)+$min),((($z+1)*$c)+($min-1)),0); foreach ($x as $val) { if (($val >= $class{$z}[0]) &amp;amp;amp;&amp;amp;amp; ($val <= $class{$z}[1])) { $class{$z}[2]++; } } } $maxCount = 0; for ($z = 0; $z<$numClasses; $z++) { if ($class{$z}[2] > $maxCount) { $maxCount = $class{$z}[2]; } } $Yscl = (500/$maxCount); $width = (70+($numClasses*70)); $histogram = imagecreate($width,560); //Defining our colors here...you remember how to do that, right? $white = imagecolorallocate($histogram,255,255,255); $black = imagecolorallocate($histogram,0,0,0); $blue = imagecolorallocate($histogram,0,0,255); imageline($histogram,50,10,50,510,$black); imageline($histogram,50,510,$width,510,$black); ... ?>

Next, we’ll handle a quick, rather minor part, labeling the histogram using the imagestring() function. There are several functions available with the GD library, including imagefttext(), imagepstext(), and imagettftext(), so pick the one that’s best for you. An accomplice to this function, we will also use imageloadfont() to add a basic font of our own to use.

<?php $font = imageloadfont('font.gdf'); imagestring($image,$font,$x,$y,"Text",$color); ?>

There are several specifications for the font file that you load, so I would read up on the specifications depending on the version of GD you have and what you want to accomplish. We aren’t going to add any text to the chart yet, but I’ll add that on in the following code. Finally, let’s actually get on to the most important part, creating the bars, using the imagefilledrectangle() function:

<?php imagefilledrectangle($image,$x1,$y1,$x2,$y2,$color); ?>

The two coordinates that are needed should be opposite corners (upper-left and bottom-right,upper-right and bottom-left, etc), though it is not important which two. As you should know by now, $image refers to the image you create and $color to a predefined color that the rectangle will be filled with. Computing the sizes of each rectangle and adding in the proper labels, here’s the additional code:

<?php imageloadfont('arial.gdf'); imagestring($histogram,$font,(($width/2)-25),527,"Classes",$blue); for ($z=0; $z<$numClasses; $z++) { imagefilledrectangle($histogram,(70+(69*$z)),509,((69*$z)+120),(509-($Yscl*$class{$z}[2])),$blue); imagestring($histogram,$font,(67+(69*$z)),511,$class{$z}[0]."-".$class{$z}[1],$black); imagestring($histogram,$font,30,(509-($Yscl*$class{$z}[2])),$class{$z}[2],$blue); imageline($histogram,47,(509-($Yscl*$class{$z}[2])),53,(509-($Yscl*$class{$z}[2])),$black); }

As you can see, we used the imagestring() function to label the x-axis, label each class of the histogram, and label the necessary heights on the y-axis to make it easy-to-read.

All-in-all, the code I use reads as follows, and I think you will see that with exception perhaps to adjusting the coordinates to fit with the image, it is not too complicated.

<?php $x = ;//Array of x-values $numClasses = ;//Desired number of classes $min = (float)$x[0]; $max = (float)$x[0]; foreach ($x as $val) { if ((float)$val<$min) { $min = $val; } if ((float)$val>$max) { $max = $val; } } $c = ceil((($max-$min)/$numClasses)); //Width of each class for ($z=0; $z<$numClasses; $z++) { $class{$z} = array((($z*$c)+$min),((($z+1)*$c)+($min-1)),0); foreach ($x as $val) { if (($val >= $class{$z}[0]) &amp;amp;amp;&amp;amp;amp; ($val <= $class{$z}[1])) { $class{$z}[2]++; } } } $maxCount = 0; for ($z = 0; $z<$numClasses; $z++) { if ($class{$z}[2] > $maxCount) { $maxCount = $class{$z}[2]; } } $Yscl = (500/$maxCount); $width = (70+($numClasses*70)); $histogram = imagecreate($width,560); $white = imagecolorallocate($histogram,255,255,255); $black = imagecolorallocate($histogram,0,0,0); $blue = imagecolorallocate($histogram,0,0,255); $font = imageloadfont('arial.gdf'); imageline($histogram,50,10,50,510,$black); imageline($histogram,50,510,$width,510,$black); imagestring($histogram,$font,(($width/2)-25),527,"Classes",$blue); for ($z=0; $z<$numClasses; $z++) { imagefilledrectangle($histogram,(70+(69*$z)),509,((69*$z)+120),(509-($Yscl*$class{$z}[2])),$blue); imagestring($histogram,$font,(67+(69*$z)),511,$class{$z}[0]."-".$class{$z}[1],$black); imagestring($histogram,$font,30,(509-($Yscl*$class{$z}[2])),$class{$z}[2],$blue); imageline($histogram,47,(509-($Yscl*$class{$z}[2])),53,(509-($Yscl*$class{$z}[2])),$black); } header("Content-type: image/png"); imagepng($histogram); imagedestroy($histogram); ?>

With this, a great-looking histogram is achieved, and the colors and text can easily be modified to combine for the look desired. PHP and the GD library once again save the day, making it easier for statisticians to graphically represent data.

Check out my sample script, where you can enter a set of data and a number of classes (can be random) and be presented a basic histogram.