statistics: memory plugin - improve graph by better scaling of y-axis

Utilise alt_autoscale_max to make the memory chart y-axis to scale better
for devices with e.g. 128 MB RAM.

Also fix the axis min value to 0.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

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

@@ -17,6 +17,8 @@ function rrdargs( graph, plugin, plugin_instance, dtype )
 		title = "%H: Memory usage",
 		vlabel = "MB",
 		number_format = "%5.1lf%s",
+		y_min = "0",
+		alt_autoscale_max = true,
 		data = {
 			instances = { 
 				memory = { "free", "buffered", "cached", "used" }