Well let's go back to the example of gold. Estimates suggest something like 170,000 tons of gold have been mined in the history of human civilization. In 2015, ~3,200 tons of new gold was pulled out of the earth. That's ~1.8% growth in the total gold supply per year. Does that make gold a poor store of value?
I agree this way of visualizing the data means that people don't see the overall growth of the cryptocurrency market, but that's kind of the point. There's a fair bit of correlation in fluctuations of value across all the different cryptocurrencies. If the price of every cryptocurrency drops by 50%, or the price of every cryptocurrency triples, that doesn't have any effect, positive or negative, on the market dominance of bitcoin.
By taking the overall fluctuations in the value of cryptocurrencies out of the picture, it is easier to see how much the relative sizes of different currencies can change over different timescales, which is the more relevant piece of information if we're trying to speculate about what the minimum amount of time it would take for an altcoin to surpass bitcoin might be.
Regarding gold, true, but the demand for gold has far out paced supply. That is what has driven the price for gold up, demand not its restricted supply. I'm not disagreeing that in a market such as that, you'd have a good store of value at hand. Likewise with Bitcoin at the moment. The bitcoin supply grew by about 6.5% over the last year (far greater than gold's), but because demand has far out paced the supply growth, we've seen huge increases in price. But, none of this paints the whole picture for both gold or bitcoin. The problem is that crypto-currencies have no theorectical maximum to the number of newly "minted" coins that could be created while there only exists so much gold in the world. So the concern that I was getting at is where you'd have a point in time where demand stagnates while supply continues to grow even if it is at a slow pace. That's where you'd end up having an inflationary currency at hand. Sure, that likely wouldn't happen with crypto-currency until we're long dead, but it is something that absolutely would need to be hard-coded into the currency from the beginning to ensure that no manipulation can occur down the road. That's why bitcoin is designed the way it is.
As far as the chart goes, I disagree. Ignoring the actual values of the market caps of each currency and only focusing on the percentages paints the wrong picture. Let me explain by bringing the extremes into play using two scenarios:
Scenario #1:
Currency A has a market capitalization of $2 million and grew 20% YTD
Currency B has a market capitalization of $1 million and grew 50% YTD
Scenario #2:
Currency A has a market capitalization of $2 trillion and grew by 20% YTD
Currency B has a market capitalization of $1 trillion and grew by 50% YTD
In scenario #1, you have an extremely early market that has yet to see much investment. In this scenario it could easily be determined that currency B could have a high probability of surpassing currency A and currency A has a high chance that it might not even exist if competition from currency B stays firm. In this scenario, there is no telling where the market will go in the future and either currency could exist or not.
In scenario #2, you have a well establish market with huge amounts of investments that have taken place. The maturity of the marketplace is well establish at this point and businesses, governments and individuals have significant investments in both currencies. Even though currency B has made tremendous gains, in this scenario, the likely outcome is that both currencies A and B will likely coexist together. Currency A, even if it doesn't stay the leader, it will still continue to have an existence in the market place and at a market cap of $2 trillion, it wouldn't be going anywhere any time soon.
A chart like yours would display both scenarios the same even though the true analysis of their markets are completely different.
By using these examples at the extremes, it helps illustrate my point. The point is that the chart is misleading with regard to the current and future crypto-currency markets. We're certainly beyond scenario #1, but we're still quite a ways off from scenario #2. However, the closer and closer we get to scenario #2 and further we get from #1, the harder it will be for any other currency to not just surpass bitcoin, but to cause bitcoin to drop from the market all together. Like I said, I firmly believe there is room in the crypto-currency market for more than one currency. The point I want to make though is that as Bitcoin gets closer and closer to scenario #2, that more likely it will be here to stay regardless of what other crypto-currencies come along.