odd plotting characters

[mariev]

system

R 2.4.1 (64-bit)
Qt 4.6.2

VGA compatible controller: ATI Technologies Inc Redwood [Radeon HD 5600 Series]
Subsystem: Hewlett-Packard Company Device 1448
Flags: bus master, fast devsel, latency 0, IRQ 35
Memory at c0000000 (64-bit, prefetchable) [size=256M]
Memory at d4000000 (64-bit, non-prefetchable) [size=128K]
I/O ports at 4000 [size=256]
Expansion ROM at d4040000 [disabled] [size=128K]
Capabilities:
Kernel driver in use: fglrx_pci
Kernel modules: fglrx

ubuntu 10.04 lts

problem

odd characters at tick mark locations. looks like ... in rainbow instead of line segment