I changed the type of chart, from a 3-D to a 2-D
Aha. Don't use 3D chart types. They are less flexible that 2D types, and
what's more, they can be deceptive.
Re your second question. Charts with dual axes can also be deceptive. If you
were to proceed anyway, make a chart with two series, as a column chart.
Right click the cumulative series, choose the chart type command, and change
it to a line type (you could have started with a line chart and changed the
annual amounts to a column, doesn't matter). Then right click the cumulative
series, choose the Format command, and change it to the secondary axis.
Your specific data is probably going to be hard to misrepresent in a dual
axis chart. For some data sets, a less unclear, less deceptive approach
would be to simply make two charts, or make a panel chart:
http://peltiertech.com/Excel/ChartsHowTo/PanelUnevenScales.html
- Jon