The four prices inside one bar
Every bar or candle encodes open, high, low and close for its interval. In a bar chart the vertical line spans high to low, a tick on the left marks the open and a tick on the right marks the close. In a candle the body spans open to close and is filled or coloured to show which came first, with thin wicks reaching to the high and low.
Both notations carry identical information. Neither records the order in which the high and low occurred, or how long the price spent anywhere. A long bar tells you the range was wide; it cannot tell you whether the price went up then down, or down then up, and treating a shape as a narrative is reading in something the data does not hold.
What volume adds, and what it does not
Volume is the number of shares traded in the interval, and it is the one honest measure of participation on the chart. A price move on unusually high volume involved more transacted shares than one on thin volume. That is all it says, and it is genuinely useful because thin-volume moves are easier to produce.
What volume does not say is who was buying. Every trade has a buyer and a seller in equal size, so volume cannot be split into buying and selling pressure; the balance is inferred by convention rather than measured. Volume is participation, not direction.
The bar that is not what it looks likeA single enormous bar with little volume behind it often reflects a thin market rather than an event; one trade at a stale price can set a high or low for the day. On thinly traded securities the chart is drawn from very few decisions, and it should be read that way.
Scale, gaps and splits distort the same series
A linear axis gives equal space to equal dollar moves; a logarithmic axis gives equal space to equal percentage moves. Over a long history and a large price change these produce visually different charts of identical data, and the log axis is the honest one for compounding series because a move from 10 to 20 and one from 100 to 200 are the same event proportionally.
Gaps occur where no trading happened between one interval's close and the next's open: overnight, or across a halt. Splits and dividends require the whole history to be adjusted, and an unadjusted chart shows a cliff where no value was lost. Before reading anything into an old chart, check whether it is adjusted; the answer is usually in a small note nobody looks at.
What a chart cannot contain
It has no information about the business. Every input is price and volume, so a chart of a company with rising revenue and one about to be delisted look the same if the price series matches. It also contains no information about the future, which is not a criticism but a statement about the data: a chart is a complete record of what has happened and no record at all of what will.
That is why this page sits in a screening section rather than a strategy one. A chart is a way of seeing a series quickly, and it belongs alongside the statements, which contain what the chart discarded.