I think that you should be talking to a statistician.
Excel has some ability to extend series.
For example if you have a standard line chart (graph), right-click on the series line and select 'Add Trendline ..."
Then you can select an algorithm that best fits your data.
Once the trendline is in place, right-click it and select 'Format Trendline' In the Options tab, you can then extend the trendline to future or
paste past periods.
Regarding a 'Bell curve', my understanding is that this was a plot of existing data, and shows whether the data has a 'Normal' or Gaussian distribution. It isn't something that can be extended. You are plotting the number of measurements that occur for any given value or range of values.
Adding more real data points will improve the accuracy of the plot, but adding calculated data isn't appropriate, unless the plot is being prepared for demonstration purposes. But again, I think that you need to talk to a statistician - which I am not.