Это то, что вы имели в виду?
variable_x <- c("population", "timepoint", "endpoint")
cat(sprintf("digraph{
graph[layout='dot',outputorder=edgesfirst,overlap=T,rankdir=LR]
b[label='%s=BARI_POP4_5_PRIMARY_CN.csv']
c [label='timepoint=12']
d[label='endpoint=ACR50']
b->c[label='']
c->d[label='']
}", variable_x[1])
)
Выход:
digraph{
graph[layout='dot',outputorder=edgesfirst,overlap=T,rankdir=LR]
b[label='population=BARI_POP4_5_PRIMARY_CN.csv']
c [label='timepoint=12']
d[label='endpoint=ACR50']
b->c[label='']
c->d[label='']
}