`<?php
include ("../jpgraph.php");
include ("../jpgraph_scatter.php");
$polex = 6;
$poley = 40;
function FldCallback($x,$y,$a) {
GLOBAL $polex, $poley;
$maxr = 3000;
// Size and arrow size is constant
$size="";
$arrowsize="";
// Since we have different scales we need the data points
// to be of the same magnitude to give it a distance
// interpretation.
$x *= 10;
// Colors gets colder the further out we go from the center
$r = ($x-$polex*10)*($x-$polex*10)+($y-$poley)*($y-$poley);
$f = $r/$maxr;
if( $f > 1 ) $f=1;
$red = floor((1-$f)*255);
$blue = floor($f*255);
$color = array($red,0,$blue);
//echo "x=$x, y=$y, blue=$blue, red=$red<br>";
return array($color,$size,$arrowsize);
}
// Create data for a simulated pseudo-magnetic radient field
$datax = array();
$datay = array();
$angle = array();
for($x=1; $x < 10; ++$x ) {
for($y=10; $y<100; $y += 10) {
$a = -1;
if( $x==$polex && $y==$poley ) continue;
if( $x==$polex ) {
if( $y > $poley ) $a=90;
else $a = 270;
}
if( $y==$poley ) {
if( $x > $polex ) $a=0;
else $a=180;
}
if( $a == -1 ) {
$d1 = $y-$poley;
$d2 = ($polex-$x)*20;
if( $y < $poley ) $d2 *= -1;
$h = sqrt($d1*$d1+$d2*$d2);
$t = -$d2/$h;
$ac = acos($t);
if( $y < $poley ) $ac += M_PI;
$a = $ac * 180/M_PI;
}
$datax[] = $x;
$datay[] = $y;
$angle[] = $a;
}
}
// Setup the graph
$graph = new Graph(300,200);
$graph->SetScale("intlin",0,100,0,10);
$graph->SetMarginColor('lightblue');
// ..and titles
$graph->title->Set("Field plot");
// Setup the field plot
$fp = new FieldPlot($datay,$datax,$angle);
// Setup formatting callback
$fp->SetCallback('FldCallback');
// First size argument is length (in pixels of arrow)
// Second size argument is roughly size of arrow. Arrow size is specified as
// an integer in the range [0,9]
$fp->arrow->SetSize(20,2);
$fp->arrow->SetColor('navy');
$graph->Add($fp);
// .. and output
$graph->Stroke();
?>
`