Here is a very simple example of what I talk about. The gradient in each part of the graph follows the direction of the circle. In other words the gradient isn't linear or radial, which are the only options to choose from in Illustrator.
The graph below has 28 segments and is applied in a more illustrative kind of way. It's still not a complex graph, but without a rather smart and simple way to create these gradients it would take a lot of time. Let's have a look on how I got started...
Draw a rectangle
Start by selecting the Rectangle tool and draw a small rectangle. Give it a color fill. The color you are choosing will be starting color of the gradient you'll create.
Duplicate the rectangle
Duplicate the rectangle by selecting the Selection tool (black arrow), and start dragging to the right as shown in the image above. While dragging, hold down the Option/Alt + Shift key. Then release the mouse first, then the keys.
Give the second rectangle the ending color of your gradient.
To prepare our perfect gradient using the blend feature, we first check our blend options. Go to Object > Blend > Blend Options. Select Smooth Color from the Spacing dropdown menu. This option will guarantee a smooth color gradient between the 2 colors. Illustrator will calculate the steps needed for a smooth blend.
Now that the options are all set, select both rectangles and go to Object > Blend > Make (or press Cmd/Ctrl + Option/Alt + B) to create the blend. If you select the blend object, you can still edit the Blend Options via the Object menu. You can also still adjust the starting and ending color if you select one of the rectangles using the Direct Selection tool (or white arrow).
Create new Art Brush
Now that we've created our perfect color blend, we are going to turn it into an Art Brush. Select the object and drag it into the Brush palette.
Select Art Brush from the brush type options
Give the brush a name. The rest of the options is left as default (see image above).
Draw a circle
Now draw a circle using the Ellipse tool holding down the Shift key while dragging.
Apply gradient Art Brush
Select the circle and choose the art brush you've just created from the Brush palette.
Adjust stroke weight
The weight of the stroke might not be to your liking. To adjust this, go to the Appearance palette and select another value from the stroke weight dropdown menu.
Start building your graph
I hope you enjoyed this little tip, and hopefully it will give you new ideas to explore the possibilities to use these kind of brushes. Some of you might still wonder "now what?". In a next tutorial I'll show you how I started building the graph. So stay tuned :)