Let average previous plots start their y axis at -1

This simplifies handling of only part-time active clients

scripts/analysis/preview-results.py

@@ -73,7 +73,7 @@ def generate_plot(outputfn, data_lines, modeline=SVG_MODELINE):
                      + "set key horizontal center bmargin\n" \
                      + "set key font \",7\"\n" \
                      + "set xrange [0:]\n" \
-                     + "set yrange [0:]\n" \
+                     + "set yrange [-1:]\n" \
                      + "set output \"{0}\"\n" \
                      + "plot {1}\n"
     data_join = ", \\\n     "