Hankmanhub, you are quite right with respect to drawing lines. An incorrectly drawn line may lead to incorrect conclusions. Drawing lines need to be objective to avoid bias. I previously posted the methodology I use to draw line, see link.
As to my predictions on price movements, I use the 60 minute charts for short term of 1 - 3 days and the daily charts for a week or more. Apart from the trend (up, down, sideways), there are technical indicators like momentum, cycles, resistance /support and many more that helps form an opinion. Patterns like bear/bull consolidation , accumulation and distribution (selling) give a visual of what the next likely move. e.g I posted a distribution pattern of NVCR around 215 and a target of 130 which was achieved last week. ( this was based on a weekly chart)
All in all, I combine the pieces of time frame, structure, volume, TA indicators and patterns into what I think is the likely direction or price target. It would not be fair if I don't put my predictions on the line so I lay it out for all to see and not claim after the fact. If I'm wrong I'm wrong. Also intuition /experience comes into play and 90 % I trust my instincts. It's like driving a car that after a while it becomes second nature.
Some people are really good with fundamentals and don't need a chart or use it to confirm one's view. eg. Jim Rogers who is great on Macro views but quite often off timing wise .