This can happen when your data is shaped in such a way that you have multiple fields that represent a single measure.
In this case, you have a separate measure for each Ward, when you really want to have a single measure (rate) per Ward (which should be a dimension).
There are a couple of alternatives:
1. use Measure Names / Measure Values. If you put Measure Names on Rows and Measure Values on Columns, you'll have a chart similar to your desired. You can copy Measure Values to Rows and then use the drop down menu on the field to switch it to discrete.
2. Reshape your data from
Month Ward 1 Ward 2 Ward 3 Jan 0.5 0.1 0.2 Feb 0.4 0.5 0.5 Mar 0.3 0.3 0.3 Apr 0.3 0.2 0.2 May 0.5 0.3 0.3
(this is "wide" data - many columns for a single measure)
Month Ward Rate Jan 1 0.5 Jan 2 0.1 Jan 3 0.2 Feb 1 0.4 Feb 2 0.5 Feb 3 0.5 ... ... ...
(this is "tall" data - many rows with one dimension and one measure for all the columns above)
You'll find the "tall" data much easier to work with in Tableau.