add diffhist struct, update expandednet widget

widgets/hist.go

@@ -44,3 +44,37 @@ func (h FloatHistData) Append(val float64) {
 	}
 	h.data = append(h.data, val)
 }
+
+type DiffHistData struct {
+	HistData
+	data    []int
+	srcData []int
+}
+
+func NewDiffHistData(max int) DiffHistData {
+	return DiffHistData{
+		NewHistData(max),
+		make([]int, max),
+		make([]int, max),
+	}
+}
+
+// return most recent value
+func (h DiffHistData) Last() int {
+	return h.data[len(h.data)-1]
+}
+
+func (h DiffHistData) Append(val int) {
+	if len(h.data) >= h.maxLen {
+		h.data = append(h.data[:0], h.data[1:]...)
+	}
+	if len(h.srcData) >= h.maxLen {
+		h.srcData = append(h.srcData[:0], h.srcData[1:]...)
+	}
+
+	diff := val - h.srcData[len(h.srcData)-1]
+	if diff != val { // skip adding to data if this is the initial update
+		h.data = append(h.data, diff)
+	}
+	h.srcData = append(h.srcData, val)
+}