ggplot: Force geom_line through origin (0,0)

I have several plots, for which I create linear geom_line models, however, I wish to force the geom_line through (0,0) and I can't seem to figure out how to do it.

Im using ggplot on a dataframe, and then using geom_line to create two linear correlations for two samples within the dataframe. With the following code

ggplot(df_test,aes(x=xval, y=yval)) +
  geom_point(aes(color = Samples)) +
  scale_color_manual(values = c("Sample1" = 'black','Sample2'='red')) +
  geom_smooth(data=df_test[df_test$Samples == "Sample1",],
              method=lm,color="black",formula= (y~poly(x,1)),se=F) +
  geom_smooth(data=df_test[df_test$Samples == "Sample2",],
              method=lm,color="red",formula= (y~poly(x,1)),se=F) +
  geom_text(aes(label=Chromosome,color=Samples),hjust=0.2, vjust=-0.5) +
  labs (y="yval") +
  labs(x="xval") +
  theme_bw()

Which yields the following image

Ideally the black line should be forced to pass through (0,0)

And I've tried to use something similar to Force the origin to start at 0

scale_x_continuous(breaks = seq(0, 12, 1)

scale_x_continuous(expand = c(0, 0)) + scale_y_continuous(expand = c(0, 0))

But it doesnt work. Is it even possible with ggplot? If so can anyone help with this problem? Thanks a lot