regenerate all history graph with an area plot, fixing scale

howto/upgrades.md

@@ -44,8 +44,6 @@ different Debian releases:
    and counting)
  * bookworm (running 18 month and counting)
 
-Note that the above X axis is inaccurate: it's scaled according to the
-number of samples, not to the *time* of the samples.
 </figcaption>
 </figure>
 

howto/upgrades/Makefile

@@ -2,7 +2,7 @@
 PROG=predict-os
 
 data.png: data.csv
-	$(PROG) graph -o $@ --path $< --plot-type barplot
+	$(PROG) graph -o $@ --path $< --plot-kind area
 
 # order of requisites matter here, the releases must be in
 # chronological order, otherwise the resulting data.csv file will be