Chart 3 - issue with nulls and ratio value on the last column

Chart 3 shown at the end of the 'Third Chart' session looks different to the chart shown at the beginning of 'Creating and Formatting a Dashboard' session. See below - specifically the last column, Feb-2018 (ratio value/column height and nulls). Can you provide steps to rectify this please and explain why this happened. Many thanks. :) P.S. I have tried filtering out the nulls but the column height stays the same with the ratio value (column height) being 0.118 instead of 0.1337.

Screenshot taken at the end of 'Third Chart' session:

Screenshot taken at the beginning of 'Creating and Formatting a Dashboard' session: