luci-app-statistics: Diskfree graph show also percent view

The Diskfree (df) plugin could also collect the values in percent if the
option "ValuesPercentage" is set in the collectd configuration.
This commit will check if "df_complex" / "percent_bytes" or both are
collected by collectd and so will show the corrsponding graph.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

applications/luci-app-statistics/luasrc/statistics/rrdtool/definitions/df.lua

@@ -5,7 +5,7 @@ module("luci.statistics.rrdtool.definitions.df", package.seeall)
 
 function rrdargs( graph, plugin, plugin_instance, dtype )
 
-	return {
+	local df_complex = {
 		title = "%H: Disk space usage on %pi",
 		vlabel = "Bytes",
 		number_format = "%5.1lf%sB",
@@ -36,4 +36,51 @@ function rrdargs( graph, plugin, plugin_instance, dtype )
 			}
 		}
 	}
+
+	local percent_bytes = {
+		title = "%H: Disk space usage on %pi",
+		vlabel = "Percent",
+		number_format = "%5.2lf %%",
+
+		data = {
+			instances = {
+				percent_bytes = { "free", "used", "reserved" }
+			},
+
+			options = {
+				percent_bytes_free = {
+					color = "00ff00",
+					overlay = false,
+					title = "free"
+				},
+
+				percent_bytes_used = {
+					color = "ff0000",
+					overlay = false,
+					title = "used"
+				},
+
+				percent_bytes_reserved = {
+					color = "0000ff",
+					overlay = false,
+					title = "reserved"
+				}
+			}
+		}
+	}
+
+	local types = graph.tree:data_types( plugin, plugin_instance )
+
+	local p = {}
+	for _, t in ipairs(types) do
+		if t == "percent_bytes" then
+			p[#p+1] = percent_bytes
+		end
+
+		if t == "df_complex" then
+			p[#p+1] = df_complex
+		end
+	end
+
+	return p
 end