How can a X-coin market cap go down while its value goes up? 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
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).