Ah, I understand now. You can have a timelion chart on a dashboard where the x-axis date field is different than the default date field for the time picker in the upper right, but the interval will always be the same. I don't believe there is any way to accomplish this today, where you can set the time field in a timelion chart, and the time interval for it to display.
You can see here I have two different charts using two different fields for the x axis, but because the interval is still dictate by the upper right time picker, they won't show results at the same time.
Your request sounds similar to this issue, which was closed: https://github.com/elastic/timelion/issues/126