{{/*
  Calculate trend percentage from sparkline data

  Usage: {{ partial "calc-trend.html" (dict "sparkline" $sparklineData) }}

  Returns: percentage change from first to last value (or 0 if insufficient data)
*/}}
{{- $sparkline := .sparkline -}}
{{- $trend := 0.0 -}}

{{- if and $sparkline (gt (len $sparkline) 1) -}}
  {{- $first := index $sparkline 0 -}}
  {{- $last := index $sparkline (sub (len $sparkline) 1) -}}
  {{- if and $first (gt $first 0) -}}
    {{- $trend = mul (div (sub $last $first) $first) 100.0 -}}
  {{- end -}}
{{- end -}}

{{- return $trend -}}