BTC ETH vs BTC - $100 in Each: 829.87
The same money into both on the first day of the window. Two lines, one question: which one actually paid, and by how much.
Both coins set to 100 on the first day shown, then tracked by their own percentage moves. Whatever range you pick, both lines start together.
For example, if ETH reads 140 and BTC 180, then a hundred dollars into each became $140 and $180 - bitcoin won by 40 points even though both made money.
The ratio chart cannot tell you this. A rising ratio inside a falling market means you lost less with ETH, not that you gained.
index = close divided by the close on the first day of the visible window, times 100, for each coin
The answer depends entirely on the start date you choose. Change the range and the winner can change with it - which is itself worth seeing.
Own compute - daily BTC closes (2010+) joined to daily ETH closes (2017+), no external API