fix: retroactively redraw trendlines when slope changes

In PineScript, line.set_xy2() redraws the entire line from the
anchor point. Our per-bar output was recording the initial flat phase
(slope=0) and only applying the slope going forward. This left 63-83%
of trendline bars with zero slope showing horizontal lines.

Fix: when the slope is first set or updated, retroactively recompute
tl_value and tl_slope for all bars from the anchor (tl_x1) forward,
matching PineScript's visual behavior. Now 97-100% of trendline bars
have correct diagonal values.

Author: MDUYN

analysis/indicators/trendline_breakout_navigator.html

@@ -256,6 +256,16 @@ def _compute_single_timeframe(
                                     tl_slope_set = True
                                 else:
                                     tl_cur_slope = slope
+
+                                # Retroactively redraw trendline
+                                # from anchor (like PineScript
+                                # line.set_xy2)
+                                for b in range(tl_x1, bar + 1):
+                                    tl_value[b] = (
+                                        tl_y1 + tl_cur_slope
+                                        * (b - tl_x1)
+                                    )
+                                    tl_slope[b] = tl_cur_slope
                             else:
                                 # Close breaks trendline at swing
                                 tl_active = False
@@ -323,6 +333,16 @@ def _compute_single_timeframe(
                                     tl_slope_set = True
                                 else:
                                     tl_cur_slope = slope
+
+                                # Retroactively redraw trendline
+                                # from anchor (like PineScript
+                                # line.set_xy2)
+                                for b in range(tl_x1, bar + 1):
+                                    tl_value[b] = (
+                                        tl_y1 + tl_cur_slope
+                                        * (b - tl_x1)
+                                    )
+                                    tl_slope[b] = tl_cur_slope
                             else:
                                 tl_active = False