Zabbixのグラデーションラインは値域が負の時のことを考えていない ref: http://qiita.com/komeda-shinji/items/0591f495cda8ff1c9c09

CLineGraphDraw.php

--- CLineGraphDraw.php  2014-12-16 15:38:26.000000000 +0900
+++ CLineGraphDraw.php  2015-04-03 22:04:19.736847627 +0900
@@ -2245,6 +2245,10 @@
                }
                break;
            case GRAPH_ITEM_DRAWTYPE_GRADIENT_LINE:
+               if ($y1 == $y2 && $y1 == $zero)
+               {
+                   break;
+               }
                imageLine($this->im, $x1, $y1, $x2, $y2, $avg_color); // draw the initial line
                imageLine($this->im, $x1, $y1 - 1, $x2, $y2 - 1, $avg_color);

@@ -2271,9 +2275,15 @@
                    $Yincr = ($diffX > 0) ? (abs($y2 - $y1) / $diffX) : 0;

                    $gy = ($y1 > $y2) ? ($y2 + $Yincr * $i) : ($y2 - $Yincr * $i);
+                   if ($gy <= $zero)
+                   {
                    $steps = $this->sizeY + $this->shiftY - $gy + 1;

                    for ($j = 0; $j < $steps; $j++) {
+                       if (($gy + $j) > $zero)
+                       {
+                           break;
+                       }
                        if (($gy + $j) < ($this->shiftY + $startAlpha)) {
                            $alpha = 0;
                        }
@@ -2284,6 +2294,24 @@
                        $color = imagecolorexactalpha($this->im, $red, $green, $blue, $alpha);
                        imagesetpixel($this->im, $x2 + $i, $gy + $j, $color);
                    }
+                   } else {
+                   $steps = $gy - $zero + 1;
+
+                   for ($j = 0; $j < $steps; $j++) {
+                       if ($gy - $zero - $j > $this->shiftY + $startAlpha) {
+                           $alpha = 0;
+                       }
+                       else {
+                           $alpha = 127 - abs(127 - ($alphaRatio * (2*$zero - $gy + $j - $this->shiftY - $startAlpha)));
+                       }
+                       $color = imagecolorexactalpha($this->im, $red, $green, $blue, $alpha);
+                       imagesetpixel($this->im, $x2 + $i, $gy - $j, $color);
+                       if (($gy - $j) < $zero)
+                       {
+                           break;
+                       }
+                   }
+                   }
                }
            break;
        }