Renders a stacked bar chart using Chart.js v2, grouping data by x-axis category and label.
BigTable — set as ReportType in def_report.
def Render(self, Param1="", Param2="", Param3="") -> str:
Returns an HTML <canvas id="chart"> element and an inline <script> that initialise a Chart.js v2 stacked bar chart. The query must return three columns per row: x-axis category (column 1), dataset label (column 2), and numeric value (column 3). Each unique label becomes a dataset; each unique category becomes an x-axis tick. Dataset colours are assigned randomly from a large predefined colour palette.
Requires chart.min.js. Uses Chart.js v2 API with stacked x and y axes. Colours are selected using random.randrange at render time.