Make a duplicate of the top and flop it vertically as shown here. With the shape selected, select Convert to Editable Shapes (Arrange menu), then select Ungroup (Arrange menu). This step is necessary to deactivate the envelope which otherwise remains editable and would prevent the next step.
Delete the 1999 text. Select the highlight, then the Transparency Tool, and change the angle as shown. The transparent end should be at the top and the transparent end at the bottom.