Include more plot points by using fillna()

Thank you, @austinbock, for pointing out that the graph could include a
few more data points from back in the era when a year could include all
men or all women in one of the slots!  Fixes brandon-rhodes#29.

Author: brandon-rhodes

Solutions-4.ipynb

@@ -601,7 +601,7 @@
     "c = cast\n",
     "c = c[c.n <= 3]\n",
     "c = c.groupby(['year', 'type', 'n']).size()\n",
-    "c = c.unstack('type')\n",
+    "c = c.unstack('type').fillna(0)\n",
     "r = c.actor / (c.actor + c.actress)\n",
     "r = r.unstack('n')\n",
     "r.plot(ylim=[0,1])"