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Fuente: http://blog.network-outsourcing.de/2015/06/19/gradient-support-for-cacti-graphs/

Agregar funciones al final del archivo rrd.php.

function gradient($vname=FALSE, $start_color='#0000a0', $end_color='#f0f0f0', $label=FALSE, $steps=20, $lower=FALSE, $alpha='FF'){
        $label = preg_replace("/'/","",$label);
        if(preg_match('/^#?([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})/i',$start_color,$matches)){
            $r1=hexdec($matches[1]);
            $g1=hexdec($matches[2]);
            $b1=hexdec($matches[3]);
        }
        if(preg_match('/^#?([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})/i',$end_color,$matches)){
            $r2=hexdec($matches[1]);
            $g2=hexdec($matches[2]);
            $b2=hexdec($matches[3]);
        }
        $diff_r=$r2-$r1;
        $diff_g=$g2-$g1;
        $diff_b=$b2-$b1;
        $spline =  "";
        $spline_vname = "var".substr(sha1(rand()),1,4);
        $vnamet = $vname.substr(sha1(rand()),1,4);
        if(preg_match('/^([0-9]{1,3})%$/', $lower, $matches)){
            $lower   = $matches[1];
            $spline .= sprintf("CDEF:%sminimum=%s,100,/,%d,* ".RRD_NL, $vnamet, $vname, $lower);
        }elseif(preg_match('/^([0-9]+)$/', $lower, $matches)){
            $lower   = $matches[1];
            $spline .= sprintf("CDEF:%sminimum=%s,%d,- ".RRD_NL, $vnamet, $vname, $lower);
        }else{
            $lower   = 0;
            $spline .= sprintf("CDEF:%sminimum=%s,%s,- ".RRD_NL, $vnamet, $vname, $vname);
        }
        for ($i=$steps; $i>0; $i--){
            $spline .=  sprintf("CDEF:%s%d=%s,%sminimum,-,%d,/,%d,*,%sminimum,+ ".RRD_NL,$spline_vname,$i,$vname,$vnamet,$steps,$i,$vnamet);
        }
        // We don't use alpha blending for the area right now
        $alpha = 'ff';
        for ($i=$steps; $i>0; $i--){
            $factor=$i / $steps;
            $r=round($r1 + $diff_r * $factor);
            $g=round($g1 + $diff_g * $factor);
            $b=round($b1 + $diff_b * $factor);
            if (($i==$steps) and ($label!=FALSE) and (strlen($label)>2) ){
                $spline .=  sprintf("AREA:%s%d#%02X%02X%02X%s:\"%s\" ".RRD_NL, $spline_vname,$i,$r,$g,$b,$alpha,$label);
            }else{
                $spline .=  sprintf("AREA:%s%d#%02X%02X%02X%s ".RRD_NL, $spline_vname,$i,$r,$g,$b,$alpha);
            }
        }
        $spline .=  sprintf("AREA:%s%d#%02X%02X%02X%s ".RRD_NL, $spline_vname,$steps,$r2,$g2,$b2,'00',$label);
        return $spline;
    }
 
 
    function colourBrightness($hex, $percent) {
        // Work out if hash given
        $hash = '';
        if (stristr($hex,'#')) {
                $hex = str_replace('#','',$hex);
                $hash = '#';
        }
        /// HEX TO RGB
        $rgb = array(hexdec(substr($hex,0,2)), hexdec(substr($hex,2,2)), hexdec(substr($hex,4,2)));
        //// CALCULATE
        for ($i=0; $i<3; $i++) {                 // See if brighter or darker                 if ($percent > 0) {
                        // Lighter
                        $rgb[$i] = round($rgb[$i] * $percent) + round(255 * (1-$percent));
                } else {
                        // Darker
                        $positivePercent = $percent - ($percent*2);
                        $rgb[$i] = round($rgb[$i] * (1-$positivePercent)); // round($rgb[$i] * (1-$positivePercent));
                }
                // In case rounding up causes us to go to 256
                if ($rgb[$i] > 255) {
                        $rgb[$i] = 255;
                }
        }
        //// RBG to Hex
        $hex = '';
        for($i=0; $i < 3; $i++) {
                // Convert the decimal digit to hex
                $hexDigit = dechex($rgb[$i]);
                // Add a leading zero if necessary
                if(strlen($hexDigit) == 1) {
                $hexDigit = "0" . $hexDigit;
                }
                // Append to the hex string
                $hex .= $hexDigit;
        }
        return $hash.$hex;
}
 

Editar el código sobre la linea la linea número 1375 de rrd.php

                //      if (preg_match("/^(AREA|LINE[123])$/", $graph_item_types{$graph_item["graph_type_id"]})) {
                //              $graph_item_stack_type = $graph_item_types{$graph_item["graph_type_id"]};
                //              $graph_variables["text_format"][$graph_item_id] = str_replace(":", "\:", $graph_variables["text_format"][$graph_item_id]); /* escape colons */
                        //      $txt_graph_items .= $graph_item_types{$graph_item["graph_type_id"]} . ":" . $data_source_name . $graph_item_color_code . ":" . cacti_escapeshellarg($graph_variables["text_format"][$graph_item_id] . $hardreturn[$graph_item_id]) . " ";
 
if (preg_match("/^AREA$/", $graph_item_types{$graph_item["graph_type_id"]})) {
        $graph_item_stack_type = $graph_item_types{$graph_item["graph_type_id"]};
        $graph_variables["text_format"][$graph_item_id] = str_replace(":", "\:", $graph_variables["text_format"][$graph_item_id]); /* escape colons */
 
$end_color =  colourBrightness("#" . $graph_item["hex"],-0.3); // End color is a 40% (0.4) darkened (negative number) version of the original color
        $txt_graph_items .= gradient($data_source_name,$graph_item_color_code,$end_color.$graph_item["alpha"],cacti_escapeshellarg($graph_variables["text_format"][$graph_item_id] . $hardreturn[$graph_item_id]),20,false,$graph_item["alpha"]);
        $need_rrd_nl = FALSE;
}elseif (preg_match("/^LINE[123]$/", $graph_item_types{$graph_item["graph_type_id"]})) {
        $graph_item_stack_type = $graph_item_types{$graph_item["graph_type_id"]};
        $graph_variables["text_format"][$graph_item_id] = str_replace(":", "\:", $graph_variables["text_format"][$graph_item_id]); /* escape colons */
        $txt_graph_items .= $graph_item_types{$graph_item["graph_type_id"]} . ":" . $data_source_name . $graph_item_color_code . ":" . cacti_escapeshellarg($graph_variables["text_format"][$graph_item_id] . $hardreturn[$graph_item_id]) . " ";

Configurar el nivel de degradado, de más claro a más oscuro, para el ejemplo un 30%.

$end_color =  colourBrightness("#" . $graph_item["hex"],-0.3); // 30% (0.3) oscuro.