Bar Chart - Positive and Negative Values

[650U]

Hi,

I am looking to create a Bar Chart based on the values I get from the form.
It works with positive values, but at times (since some math are done to find the value) I do have negative values and the graph just shows a bar going down that's it. It won't display values as it displays for positive values.

I am attaching the code, any help would be appreciated!

<?php
    /*Instead of pasting my entire form code I am just assinging values here and stored in array*/
	
    $a = 410;
	$b = 130;
	$c = -215;
	$d = 81;
	$e = 310;
	$f = -190;
	# ------- The graph values in the form of associative array
	$values=array($a, $b, $c, $d, $e, $f);
	

 
	$img_width=450;
	$img_height=300; 
	$margins=20;

 
	# ---- Find the size of graph by substracting the size of borders
	$graph_width=$img_width - $margins * 2;
	$graph_height=$img_height - $margins * 2; 
	$img=imagecreate($img_width,$img_height);

 
	$bar_width=20;
	$total_bars= count($values);
	$gap= ($graph_width- $total_bars * $bar_width ) / ($total_bars +1);

 
	# -------  Define Colors ----------------
	$bar_color=imagecolorallocate($img,0,64,128);
	$background_color=imagecolorallocate($img,240,240,255);
	$border_color=imagecolorallocate($img,200,200,200);
	$line_color=imagecolorallocate($img,220,220,220);
 
	# ------ Create the border around the graph ------

	imagefilledrectangle($img,1,1,$img_width-2,$img_height-2,$border_color);
	imagefilledrectangle($img,$margins,$margins,$img_width-1-$margins,$img_height-1-$margins,$background_color);

 
	# ------- Max value is required to adjust the scale	-------
	$max_value=max($values);
	$ratio= $graph_height/$max_value;

 
	# -------- Create scale and draw horizontal lines  --------
	$horizontal_lines=20;
	$horizontal_gap=$graph_height/$horizontal_lines;

	for($i=1;$i<=$horizontal_lines;$i++){
		$y=$img_height - $margins - $horizontal_gap * $i ;
		imageline($img,$margins,$y,$img_width-$margins,$y,$line_color);
		$v=intval($horizontal_gap * $i /$ratio);
		imagestring($img,0,5,$y-5,$v,$bar_color);

	}
 
 
	# ----------- Draw the bars here ------
	for($i=0;$i< $total_bars; $i++){ 
		# ------ Extract key and value pair from the current pointer position
		list($key,$value)=each($values); 
		$x1= $margins + $gap + $i * ($gap+$bar_width) ;
		$x2= $x1 + $bar_width; 
		$y1=$margins +$graph_height- intval($value * $ratio) ;
		$y2=$img_height-$margins;
		imagestring($img,0,$x1+3,$y1-10,$value,$bar_color);
		imagestring($img,0,$x1+3,$img_height-15,$key,$bar_color);		
		imagefilledrectangle($img,$x1,$y1,$x2,$y2,$bar_color);
	}
	header("Content-type:image/png");
	imagepng($img);

?>

Also attaching my output. If I understand this then I can create a Bar Chart report for my website.

Thanks!

[diafol]

I'm no GD guy, but looking at the output, your y-axis is a bit mashed. It doesn't have a linear scale and doesn't go below 0. If you could set your increments and min-value (negative) as well as max-value, I think it may work. You need to be able to represent x-axis values for negative bars (on top of the axis itself)?

[ajbest]

I had a similar need a few months ago and instead of writing my own script I found a GPU php class called pChart. This little class really has a lot of options to allow for image control.

The only downfall on this script is the processing time. For my need I had to have the image created at the time of data change as oppose to the time of user request.

Just thought I would pass on my solution.