user44978: Quoting Wikipedia, Market cap is given by the formula 
MC = N × P   
where N is the number of shares outstanding, and P is the closing price per share.

In the specific case of a crypto currency, one could assume that N is the amount of coins in circulation and P its current value. 
How can the image below happen, given that Zcash, being a fork of Bitcoin, cannot (unless I missed something very important) be destroyed, thus N can only grow.

Source: coinmarketcap.com, a few minutes before this post

user5406: As Nate Eldredge pointed out, the chart you're looking at is denominated in BTC, while the market cap is denominated in USD. 
If you look at the chart below, you'll see that the price in USD (green) behaves exactly as you describe in your question. 
On the other hand, the price in BTC (orange) does not behave like you expected, because it combines the price movement of Bitcoin with the price movement Zcash respective to the Market Cap denominated in USD (blue).


44978: <p>Quoting Wikipedia, Market cap is given by the formula </p>

<pre><code>MC = N × P   
where N is the number of shares outstanding, and P is the closing price per share.
</code></pre>

<p>In the specific case of a crypto currency, one could assume that <code>N</code> is the amount of coins in circulation and <code>P</code> its current value. </p>

<p>How can the image below happen, given that Zcash, being a fork of Bitcoin, cannot (unless I missed something very important) be destroyed, thus <code>N</code> can only grow.</p>

<p><a href="https://i.stack.imgur.com/jn7u2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jn7u2.png" alt="enter image description here"></a>
Source: coinmarketcap.com, a few minutes before this post</p>
 5406: <p>As <a href="https://bitcoin.stackexchange.com/questions/50721/how-can-a-x-coin-market-cap-go-down-while-its-value-goes-up/50723#comment58828_50721">Nate Eldredge pointed out</a>, the chart you're looking at is denominated in <code>BTC</code>, while the market cap is denominated in <code>USD</code>. </p>

<p>If you look at the chart below, you'll see that the price in <code>USD</code> (green) behaves exactly as you describe in your question. </p>

<p>On the other hand, the price in <code>BTC</code> (orange) does not behave like you expected, because it combines the price movement of Bitcoin with the price movement Zcash respective to the Market Cap denominated in <code>USD</code> (blue).</p>

<p><a href="https://i.stack.imgur.com/onRCy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/onRCy.png" alt="Zcash chart"></a></p>
