Bitcoin is not the only digital currency. An article in The Economist discusses the most popular crypto-currencies. There are now approximately 1492!
The growth of these increasing number of currencies has reduced the market share of bitcoin.
As is usual with area charts, it is easy to follow the base area and the top area, but more challenging to understand the others. Bitcoin is dropping rapidly and now well under 40%. The other basket of currencies is increasing and now amounts to approximately 30%
The data source provides an alternate presentation, an overlapping area chart.
With this display, it is easier to follow the individual coins, but the chart includes too many of them. More important, almost all area charts are the stacked variety, so this one might be misinterpreted. It would probably be better to use lines.
It can be a bit easier to see which coin is most eating away at Bitcoin be reversing the stacking order.