How to create a scatter graph of chain stores on the y-axis with a dot over each city a store is located long the x-axis?

[lyndac]

I have Splunk indexing a file that contains information about the geographical location of stores:

city, chain, numStores
Pasadena, Walmart, 0
Pasadena, Kmart, 1
Glen Burnie, Walmart, 1
Glen Burnie, Target, 1
Glen Burnie, Kmart, 1
Millersville, Target, 1

I want to be able to plot this data into a scatter chart where the X-axis is the city, the Y-axis is the chain, and the "dot" appears at the intersection where the city has a store (similar to below, but the X's are dots and they are lined up in the column):

Walmart|                       X
KMart  |      X                X
Target |                       X                  X
-----------------------------------------------------------------
           Pasadena       Glen Burnie        Millersville

Try as I might, I cannot even get the axes to display correctly. Is it possible to do what I am asking? I read in the "Data Requirments for Visualizations" document that I need to graph the events directly, so I tried this:
index=foo | fields - _* | fields city, chain (I get no results for this)
index=foo | fields city, chain (I don't see any plots, but the legend shows up with city and _time. The x axis is labeled city, but no values are displayed and the y-axis shows values 50 and 100 which are not even in the data.)

I wish I could paste the graph here, but my Splunk is on a closed instance so I have to retype everything here.

[afishkin_splunk · Accepted solution]

scatter graph returns number for both Y-Axis and X-Axis
http://docs.splunk.com/Documentation/Splunk/6.3.0/Viz/Datastructurerequirementsforvisualizations and select Scatter charts link on the right panel