[metapost] Why only the contents of the 1st quadrant left?

[Song Xiaodong]

Hi,

I have made a figure using metapost and inserted it into a latex document.
When previewed in dvi format, nothing abnormality, but when the document
changed from dvi to pdf (using dvitopdf), the contents of the left part of
Y axis and the below part of  X axis all missed. Why?

Thank you!

Song
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