Line Drawing Issue

You have to use the first and third time/price points to draw the line, with OBJPROP_RAY = 0.

Drawing to a "future" time point is permitted.

Sorry, I guess I wasn't clear enough.

The slope of the line I want to draw is defined by the price at TimeA (Start) and TimeB (End). But I want the line to extend to TimeC. I cannot use the Trendline object because it only allows 2 price/time inputs. One option, I suspect, looking more closely at the options, is to use the object type Trendbyangle. Is that correct? Since I don't know enough about what I'm doing yet, I think I might need to write a routine to draw a trendline for the first two points. Then find the angle of that line, then, redraw the line as a Trendbyangle line. The question then is, how is the angle calculated? Is it it computed by price over time, or is it computed by pixels? ... Or maybe that wouldn't matter? ... Anyway, the alternative thought is to calculate the trendline, keep it as a ray type, until TimeC. At TimeC I could do a ObjectGet on the price, and then redraw the Trendline with that as the final point, and turn the ray property off.

Just seems like al ot of manipulations to get what is pretty intuitive on the human side... which I know happens alot in the programming world. But, I just have to ask am I overlooking a simpler way? Thanks for your response.

Slope is the key factor. Knowing the origin Time/Price and the slope (pips/bar), you can plot to any time in the future (or past).

TrendlineByAngle is useless. It is not tied to the chart, it is tied to the pixels on your screen. Draw a 45 degree TrendlineByAngle.
Change time frame, change the scale, zoom in, soom out. Useless.