Use CSS for handling margins, not absolute placement in plotter

Change-Id: I88b5f23fd5b1474894661b70517873ef553f4436
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/analysis/plot_index.ts b/frc971/analysis/plot_index.ts
index 150407e..e423c55 100644
--- a/frc971/analysis/plot_index.ts
+++ b/frc971/analysis/plot_index.ts
@@ -96,7 +96,7 @@
 rootDiv.appendChild(plotSelect);
 
 const plotDiv = document.createElement('div');
-plotDiv.style.top = (plotSelect.getBoundingClientRect().bottom + 10).toString();
+plotDiv.style.marginTop = '10px';
 plotDiv.style.left = '0';
 plotDiv.style.position = 'absolute';
 rootDiv.appendChild(plotDiv);