x ticks not in the center of chromosome

[zpliu1126]

Dear Stephen,
I found the tick of x axis is not in the center of chromosome，I guess it is possible that the uneven distribution of SNP markers on chromosome. Is there an parameter to set the position of x ticks according to the real position of SNPs?

There is my sessionInfo

Output exceeds the [size limit](command:workbench.action.openSettings?[%22notebook.output.textLineLimit%22]). Open the full output data[ in a text editor](command:workbench.action.openLargeOutput?a4c9b5d7-98da-4511-a949-49b268d1cc77)
R version 3.6.0 (2019-04-26)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: CentOS Linux 7 (Core)

Matrix products: default
BLAS:   /public/home/software/opt/bio/software/R/3.6.0/lib64/R/lib/libRblas.so
LAPACK: /public/home/software/opt/bio/software/R/3.6.0/lib64/R/lib/libRlapack.so

qqman_0.1.8