150 ETH/USD is normal forex notation.

The graph shows a currency pair. So one currency is being sold in exchange for the other.

When you see currency pairs like this, ETH/USD, the first currency (ETH) is the one being bought, the second currency (USD) is the one being sold. If the chart is going up, the first currency is becoming stronger against the second. If the chart is going down, the first currency is becoming weaker against the second.

In that light it does make sense mathematically. The currency being sold is the denominator and the currency being bought is the numerator. The value of the currency being bought, the numerator is directly proportional to the exchange value of the pair. The value of the currency being sold, the denominator, is inversely proportional to the exchange value of the pair

That it took you that much to explain kind of illustrates the point, I think.

Imagine the worth of ETH and USD denominated in ISK.

    ETH in ISK = 16524.55
    USD in ISK = 103.42

    ETH / USD
    (1 * ETH) / (1 * USD)       # Multiplicative identity
    16524.55 ISK / 103.42 ISK   # Expand to ISK
    16524.55     / 103.42       # Cancellation of units
    159.78                      # Division

...is this an Eve Online reference? If so, although unexpected, I appreciate it.

Icelandic króna, more likely. AFAIK, there isn't an open exchange between currency and in-game ISK.

EVE Online was built in Iceland. It is no coincidence that the "InterStellar Kredit" and the "ÍSlensk Króna" have the same currency symbol. Iceland at one time had great aspirations in the banking sector...

It comes from the currency markets. What you really care about is the relative value of the first currency. The last or bottom currency is the base currency which is what your using to gauge the value of the first relative to other pairs with the same base.

https://en.m.wikipedia.org/wiki/Currency_pair