I am trying to fit data with a GLM (poisson regression) in R.  When I plotted the residuals vs the fitted values, the plot created multiple (almost linear with a slight concave curve) "lines".  What does this mean? 

```r
library(faraway)
modl <- glm(doctorco ~ sex + age + agesq + income + levyplus + freepoor + 
                freerepa + illness + actdays + hscore + chcond1 + chcond2,
                family=poisson, data=dvisits)
plot(modl)
```

![enter image description here][1]


  [1]: https://i.sstatic.net/7gwmX.png