Strange velocity vectors - thoughts?

[s849]

Hi all,

Thank you for developing velocyto. In the past, I have successfully overlaid velocity vectors onto a Monocle trajectory. Now, I wanted to overlaid them onto UMAP. It worked well; no issues present. However, I noticed that the vectors looked strange and different than before - they are uneven and very long (see attached). I do not see this problem when I use the same data set on Monocle.

Here's the code I used to project velocities.

show.velocity.on.embedding.cor(umap_emb,rvel,n=2000,scale='sqrt',cex=0.8,arrow.scale=40,show.grid.flow=TRUE, min.grid.cell.mass=1,grid.n=40,arrow.lwd=0.5,do.par=T,cell.border.alpha = 0.5, xlab = "Velocity component 1", ylab = "Velocity component 2") + scale_color_manual(values=c("darkgreen", "firebrick1"))

Any help will be appreciated! Thank you!