I'm not sure if you're after a trend line as opposed to just a function with a constant. (think y=mx+b from algebra class)
If you are definitely using dates (days or months) on your X axis, you might try making a calculated field as a function of number of days since a base point in time. I haven't really flushed this out, but hopefully you get my drift.
So if the goal "increases" from a starting point of 9/1/2012 by 75 each month:
DATEDIFF('month',#9/1/2012#,[DateField]) * 75 + 6800
You should be able to display the MAX or AVG of this calculated field as a Line measure and each time-point's value will be a bit higher than the previous time-point.
a few more options:
Switch from 'month' to 'day' if you want to increment the goal daily
Replace #9/1/2012# with a parameter to allow the user to dynamically set the "start point"
Replace 75 with a parameter to allow the user to dynamically set the "delta" coefficient factor of this function
Replace 6800 with a parameter to set the "goal at the base point in time" to which you add as time passes
Hope that makes sense/is helpful