How do traders actually draw a trendline?
A trendline is the simplest drawing on any chart and the most abused. Done with rules, it is a compact summary of the pace of a move. Done by eye, it is a way of proving whatever you already believed.
The mechanics
- In an uptrend, a trendline connects swing lows — the rising floor under the pullbacks.
- In a downtrend, it connects swing highs — the descending ceiling over the bounces.
Two points define a line, so two swing points are the minimum. But two points define a line through any two dots on a page, which is why most practitioners treat a line as merely drawn at two touches and only interesting once price has approached it a third time without cutting straight through it.
A trendline adds one thing the HH/HL sequence does not: slope. Structure tells you the direction; the line tells you the rate. A shallow line describes a grind; a steep one describes a sprint. What slope does not tell you is whether the pace is changing: on a log chart any straight line is a constant percentage rate — a steeper straight line is simply a higher constant rate — and only a line that bends upward describes an accelerating one. The angle in degrees means nothing at all. It changes the moment you resize the window or crop the price range, so any rule that names a specific angle is describing your screen, not the market.
A worked example
A stock puts in swing lows of $50 in January and $62 in April — roughly $4 per month of rising floor. Extend that line forward and it sits near $74 by July.
If price pulls back in July, traders watching this line will be looking at the $74 area, alongside whatever else sits there. Two things are worth saying plainly. First, the line has no power of its own: nothing physical happens at $74. What can happen is that enough participants are looking at similar lines that their orders cluster in the area — attention is the only mechanism. Second, the projection is 2 × April − January, so the two anchors do not carry equal weight: a 3% error in the April low ($1.86) moves July by $3.72, while the same 3% on the January low ($1.50) moves it by only $1.50, and in the opposite direction. Trendlines are approximations that look precise, the newest anchor carries double, and that mismatch is where overconfidence gets in.
The subjectivity problem
Hand the same chart to five traders and you'll get five different lines. The common disagreements:
- Wicks or bodies? Connecting the extreme lows gives one line; connecting the lowest closes gives a shallower one. Both are defensible. Pick one and stay with it.
- Which swings? Skip an awkward pivot and the line gets much prettier. This is the single most common self-deception in charting.
- Which scale? As Unit 1 showed, a straight line on a linear chart is a curve on a log chart. On multi-year histories this changes everything.
The professional response is not to abandon trendlines — it is to write your convention down (say: log scale, connect swing lows by wick, minimum 5-candle swings, never skip a pivot) and then apply it even when the resulting line is ugly. The ugly line is the honest one.
What a broken line is and isn't
When price closes through a trendline, the correct description is: the pace defined by those swing points no longer holds. Price may still be in an uptrend by the HH/HL test — a steep line breaking often just means the move slowed down. Traders watch line breaks as a change in rate, and structure breaks as a change in direction. Conflating the two is a classic beginner error.
Try it now
- On the schematic above, read the two anchors, work out the line's rise per bar, and project it forward to the third marked approach. Compare your number with where that bar's low actually sits. You have just done the whole of what a trendline does.
- Now pick a clear multi-month move below and draw one line the same way, writing your convention down — wicks or closes — before you start. Count how many times price came near it without cutting through.
- Deliberately draw a second, "prettier" line by skipping one inconvenient swing. That gap between the honest line and the flattering one is the bias you now know to watch for in your own work.