When to draw a range for each period instead of one value, which interval chart fits, and how to mark up and down periods so every reader can tell them apart. The same rules hold in every chart library. For picking between chart types in general, see Choosing a chart.
What it is for
These charts show a range for each period, not one value. A candlestick shows a trading day's open, high, low and close. A range band shows the daily low and high of a temperature. A percentile or volatility band shows where most values fall around a middle line. The reader sees both the level and how much it moved.
When to use
The reader trades or audits prices and needs each period's open, high, low and close. Use a candlestick or OHLC chart.
Only the move within each period matters: where it opened and where it closed. Use open to close bars.
Each period has a low and a high, such as temperatures or a price range. Use a range band.
Many values per period are summarized, and the spread matters as much as the middle. Use a percentile ribbon.
You show how volatile a series is around its moving average. Use a Bollinger band.
Use something else when
The audience wants the trend, not the daily moves. Use a line chart of the closing price.
There are hundreds of periods on a small chart. Candles shrink to lines. Use a line chart, or aggregate to weeks or months.
You compare totals per period, such as revenue per month. Use a bar chart.
You compare the spread of a few groups, not of periods over time. Use a box plot.
You compare the performance of several assets. Use a line chart of each one's change in percent from a shared start.
How it reads
A candlestick has a thin wick from the low to the high and a thick body from the open to the close. A long body is a strong move, long wicks are a day that swung and came back. An OHLC bar shows the same four values with a vertical line, a tick to the left for the open and a tick to the right for the close.
A band is read by its width and its middle. A wide band is a period of large spread or high volatility, a narrow one a calm period.
Do it well
Mark up and down periods with a second signal besides color: a hollow body for up and a filled one for down, the OHLC ticks, or a label in the tooltip.
Use the theme's positive and negative chart colors for up and down, not your own red and green, so they match the rest of the product and work in dark mode.
Use a time scale and leave gaps for missing periods, such as weekends and holidays, only when the reader expects them. Otherwise place trading days next to each other and label the dates.
Let the price axis start near the data, not at zero. These charts compare positions, not lengths.
Say what a band means in the subtitle: the 10th to the 90th percentile, or two standard deviations around a 20 day mean.
Give the wick and the body enough contrast with the background, in light and dark mode.
Pitfalls
Red and green as the only signal. About one in twelve men cannot tell them apart, and the colors mean the opposite in some markets.
Too many candles. Past about a hundred periods the bodies turn into lines and the chart becomes noise.
A band with no explanation. Readers take it for a forecast or a guarantee.
A volatility band with too few periods in its window. The band jumps with every value. Say how many periods it uses.
Inventing values for days with no trading. Leave them out, or show the gap.
Variants
Candlestick: wick from low to high, body from open to close.
OHLC bars: the same four values as a line with two ticks.
Open to close bars: one bar per period from the open to the close, with no wick.
Range band: a filled area between a low and a high line.
Percentile ribbon: a median line inside a band from a low to a high percentile.
Bollinger band: a moving average with a band of two standard deviations above and below it.
Compared with similar charts
Against a line chart: a line shows one value per period, a candlestick shows four. Use the line when the reader only follows the trend.
Against a box plot: both show a spread, but a box plot summarizes one group and these charts follow the spread through time.
Against error bars: error bars show the uncertainty of an estimate, a band shows the observed range. Say which one you draw.
Candlestick against OHLC: the same data. Candles make the body easier to see, OHLC bars stay readable without color.