Explanation
The best data for forecasting is time series data or uniformly increasing whole numbers. The line chart has to have only one line.
Try forecasting: Try the new forecasting capabilities of Power View today on your own data or with the sample report available as part of the Power BI report samples. To view your own data, upload a workbook with a Power View time series line chart to Power BI for Office 365.
Reference:
https://powerbi.microsoft.com/en-us/blog/introducing-new-forecasting-capabilities-in-power-view-for-office-365