You can try playing with the XValues of the line series so that you can force them to start a bit to the left of the bar and end a bit to the right. Also, setting AutoRange of the XAxis to false and manually selecting the min, max and step values can help with the adjustments.
We have logged a new feature request for custom trendlines which will void the need to "hack" line series for such scenarios.
We apologize for this inconvenience.
the Telerik team