fix: formular for mapping on grid points

toargridding/grids.py

@@ -166,7 +166,7 @@ class RegularGrid(GridDefinition):
         return pd.Series(id_i, index=id_x.index)
 
     def coord_to_index(self, coord, x0_axis, d_axis):
-        return (np.ceil(coord / d_axis) - x0_axis / d_axis).astype(int)
+        return (np.ceil((coord / d_axis) - 0.5) - x0_axis / d_axis).astype(int)
 
     def get_empty_grid(
         self, time: pd.DatetimeIndex, metadata: pd.DataFrame