Suppose you bought the biggest coins other than bitcoin at the start of a year and held. Each line is one of them, priced in bitcoin and indexed to 100 on that day. Falling means losing ground to bitcoin. The coins that collapsed are included; that's the point.
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In "top coins of a given year," the eight largest non-bitcoin coins by market value at the start of that year, taken from published market-cap rankings (a hand-kept list at the top of this file), each priced in bitcoin and indexed to 100 on January 1. The dashed line is an equal-weight basket of all eight, rebalanced never. Coins that were later delisted keep their last known price. Bitcoin is the flat line at 100. In "top coins today," the current rankings, refreshed on every load, with stablecoins and wrapped tokens excluded.
The "today" view has survivorship bias: it only shows coins that are still large, which flatters the group. The "given year" view fixes that, which is why it is the default. Neither view says anything about why a coin moved, and past performance is a record, not a forecast. Not investment advice.