Here's how you can do what you want:

Replace the text by numbers using a mapping scheme such as:

Remote    1
Probable    2
Possible    3
Likely    4

Put the above in some worksheet range.

Plot the graph using these numbers such that the months are along the
x-axis and the Theme forms the legend.

Next, create the data set:

0    1
0    2
0    3
0    4

Add this to the graph.  To do so, select the above range and drag onto
the chart.  In the resulting dialog box, ensure you check 'New Series'
and 'Data in Columns'.

Select this new series.  Select Chart | Chart Type... and select XY
Scatter chart.  XL will automatically add a secondary x- and y- axes.  
Double click the secondary y-axis.  From Patterns set everything to
None.  From the Scale tab, set Major Gridlines to 1 and Max. value to
5.  Format the secondary x-axis to show no pattern information.  For
the primary y-axis, set the Axis labels to None (double click the axis,
then select the Patterns tab).

If you don't already have Rob Bovey's add-in, XY Chartlabeler, get it
from www.appspro.com.  Use it to add the labels to the dummy series
added above.  Put the labels to the left.

Adjust the Plot area so that the labels show up correctly.  Select the
legend.  Pause, then select the legend for the dummy series.  Delete
it.