Add documentation for axis titles.

docs/graph2d.html

@@ -455,6 +455,18 @@ The options colored in green can also be used as options for the groups. All opt
   <td>true</td>
   <td>Show or hide the data axis.</td>
 </tr>
+<tr>
+  <td>dataAxis.title.left.text</td>
+  <td>String</td>
+  <td>undefined</td>
+  <td>Set the title for the left axis.</td>
+</tr>
+<tr>
+  <td>dataAxis.title.right.text</td>
+  <td>String</td>
+  <td>undefined</td>
+  <td>Set the title for the right axis.</td>
+</tr>
 <tr>
   <td>groups.visibility</td>
   <td>Object</td>