diff --git a/files/system-monitor-graph@mount/desklet.js b/files/system-monitor-graph@mount/desklet.js
index 13a6b63..f42755a 100644
--- a/files/system-monitor-graph@mount/desklet.js
+++ b/files/system-monitor-graph@mount/desklet.js
@@ -374,7 +374,7 @@ SystemMonitorGraph.prototype = {
                       value = hdd_use / 100;
                       break;
                   case "space":
-                      value = hdd_free / hdd_total;
+                      value = 1.0 - hdd_free / hdd_total;
                       break;
               }
               let hdd_prefix = "";