RE: Dotted line for future data

The line for actual data is solid whilst future data is display with a dashed line.

The line is actually to data series with the 2nd line formatted with the line style dashed.

Select the range A1:C10 and create a normal Line chart.
Format both series lines to give you the solid and dashed line styles.

To make sure the lines appear as one the first Future data point should be the same value as the last Actual data point.