Creating a graduated vector layer symbol renderer

A graduated vector layer symbol renderer is the vector equivalent of a raster color ramp. You can group features into similar ranges and use a limited set of colors to visually identify these ranges. In this recipe, we'll render a graduated symbol using a polygon shapefile.

Getting ready

You can download a shapefile containing a set of urban area polygons from https://geospatialpython.googlecode.com/files/MS_UrbanAnC10.zip.

Extract this file to a directory named ms in your qgis_data directory.

How to do it...

We will classify each urban area by population size using a graduated symbol, as follows:

1. First, we import the QColor object to build our color range.

   from PyQt4.QtGui import QColor
   

2. Next, we load our polygon shapefile as a vector layer:

   lyr = QgsVectorLayer("/qgis_data/ms/MS_UrbanAnC10.shp", "Urban Areas", "ogr")
   

3. Now, we build some nested Python tuples that define the symbol graduation. Each item in the tuple contains a range label, range start...