add note about matplotlib 1.4 cmap issue in scatter

Author: rasbt

code/ch02/ch02.ipynb

@@ -880,6 +880,24 @@
     "plt.show()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "#### Additional Note (3)\n",
+    "\n",
+    "The `plt.scatter` function in the `plot_decision_regions` plot may raise errors if you have matplotlib <= 1.5.0 installed if you use this function to plot more than 4 classes as a reader pointed out: \"[...] if there are four items to be displayed as the RGBA tuple is mis-interpreted as a list of colours\".\n",
+    "\n",
+    "```python\n",
+    "plt.scatter(x=X[y == cl, 0], y=X[y == cl, 1],\n",
+    "            alpha=0.8, c=cmap(idx),\n",
+    "            marker=markers[idx], label=cl)\n",
+    "```\n",
+    "\n",
+    "\n",
+    "To address this problem in older matplotlib versions, you can replace `c=cmap(idx)` by `c=colors[idx]`."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},