If I want to put an Excel forecast or projection over a Google Analytics metric, I can use 2 data sources to do it. Add the Excel file to your Dropbox and then add Dropbox as a second data source to the metric. Make sure the time frames are similar. If one report is in months, the other should be in months as well. You can use the Data Grouping function to aggregate by the time frame of the first report. Aggregation can change the columns so make sure your report is still accurate, including the key value. You can change the chart type and customize the colors to clearly differentiate between the report types.