Date Axis Spacing on Pivot Chart

I've created a pivot chart to plot a change in asset allocations (%stocks, bonds, etc.) over time. The data is collected at irregular intervals. So the time between dates isn't the same from observation to the next. When I created a pivot chart, its not spacing the data points in proportion to the # of days between observations. In formatting the x-axis, I selected date for axis type. However, that didn't seem to do anything. Any thoughts on how to fix this? If I create it as a standard chart, it works fine. But I'd like to create it as a pivot chart.