Error in title(...) : X11 font ....

Hi list,

Could anybody help me to explain the following error message and fix it?  Thank you very much!

Tao

sessionInfo() 

R version 3.1.2 (2014-10-31) 
Platform: x86_64-unknown-linux-gnu (64-bit) 

locale: 
[1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C 
[3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8 
[5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8 
[7] LC_PAPER=en_US.UTF-8       LC_NAME=C 
[9] LC_ADDRESS=C               LC_TELEPHONE=C 
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C 

attached base packages: 
[1] stats     graphics  grDevices utils     datasets  methods   base

plot(1:10, xlab="haha") 
plot(1:10, ylab="haha") 
plot(1:10, main="haha") 

Error in title(...) : 
X11 font -adobe-helvetica-%s-%s-*-*-%d-*-*-*-*-*-*-*, face 2 at size 14 could not be loaded