Themes and heatmap: a whole market compressed into one picture
A heatmap lets you compare the day's average price change across themes at a glance, but it compresses a lot into two visual variables. Knowing exactly what those two encode is what keeps you from over-reading it.
How themes are grouped
A theme is a hand-maintained membership list that changes as it is maintained. One stock can belong to several themes, and the theme tags on news items and stock pages come from the same list. It is a research grouping, not an official industry classification.
What colour and area encode
Colour always encodes the theme's average percentage change—its direction and magnitude: an equal-weight mean over members that have a quote, with missing quotes excluded rather than counted as zero. Area is switchable—by default the magnitude of that move (with a floor so a near-flat theme stays clickable), or the summed USD market capitalisation of members.
Limits when reading rotation
Equal weighting gives a small cap the same pull as an index heavyweight, so one limit-up name can lift an entire theme. In market-cap mode, members with no capitalisation or no exchange rate are excluded and the view discloses coverage instead of filling gaps with zero. A single day's colour is a snapshot; it is not proof of rotation and not advice.