Panel a: cloud distribution of the best-fit BLR model. Each circle represents one cloud colour-coded by the LOS velocity. The green ellipse at the origin illustrates the uncertainty of the offset of the BLR centre. Panel b: observed averaged differential phase from baselines UT4−UT2, UT4−UT1, and UT3−UT1 after removing the residual ‘continuum phase’ signal (blue points) compared to the averaged differential phase from the best-fit BLR model (lower red line). The black points and overlaid red line show the observed and best model line flux profile respectively. Panel c: averaged differential phase data (points) and the best-fit models (lines) of the three baselines that show the strongest signal of the BLR component (dashed lines). The phase in panel b is calculated by averaging the phases of these three baselines after subtracting the best-fit residual continuum phases (dotted lines).