sub colorRamp {
    my( $v, $vmin, $vmax ) = @_;
    my( $r, $g, $b ) = (1) x 3;
    $v = $vmax if $v > $vmax;
    $v = $vmin if $v < $vmin;

## Uncomment below to invert the color range 
## so that small numbers are hot (red) 
## and high numbers are cold (blue)
##    $v = $vmax + $vmin - $v; 

    my $dv = $vmax - $vmin;
    if( $v < ( $vmin + 0.25*$dv ) ) {
        $r = 0;
        $g = 4 * ($v - $vmin) / $dv;
    }
    elsif( $v < ( $vmin + 0.5 * $dv ) ) {
        $r = 0;
        $b = 1 + 4 * ($vmin + 0.25 * $dv - $v) / $dv;
    }
    elsif( $v < ( $vmin + 0.75 * $dv ) ) {
        $r = 4 * ($v - $vmin - 0.5 * $dv) / $dv;
        $b = 0;
    }
    else {
        $g = 1 + 4 * ($vmin + 0.75 * $dv - $v) / $dv;
        $b = 0;
    }
    ## Convert the 0->1 range rgb values to 0->255 
    ## and output as a web/tk style "#rrggbb" hex string
    return sprintf "#%02x%02x%02x", $r*255, $g*255, $b*255;
}