Diff aggregation (kalkih#601)

* Add aggregation 'diff' (= last - first)

* Add `diff` to documentation

* Rewrite delta or diff if check for readability

Co-authored-by: Mischa <mspelt@outlook.com>
Co-authored-by: Jonas De Kegel <jonas@fluid.desi>

README.md

@@ -244,6 +244,7 @@ These buckets are converted later to single point/bar on the graph. Aggregate fu
 | `last` | v0.9.0 |
 | `sum` | v0.9.2 |
 | `delta` | v0.9.4 | Calculates difference between max and min value
+| `diff` | v0.11.0 | Calculates difference between first and last value
 
 ### Theme variables
 The following theme variables can be set in your HA theme to customize the appearance of the card.

src/graph.js

@@ -15,6 +15,7 @@ export default class Graph {
       last: this._last,
       sum: this._sum,
       delta: this._delta,
+      diff: this._diff,
     };
 
     this._history = undefined;
@@ -246,8 +247,12 @@ export default class Graph {
     return this._maximum(items) - this._minimum(items);
   }
 
+  _diff(items) {
+    return this._last(items) - this._first(items);
+  }
+
   _lastValue(items) {
-    if (this.aggregateFuncName === 'delta') {
+    if (['delta', 'diff'].includes(this.aggregateFuncName)) {
       return 0;
     } else {
       return parseFloat(items[items.length - 1].state) || 0;