Hi Penyuan
Maybe you should change your choice of best answer as things have evolved. This would make this question more up to date and more helpful for other users.
Cheers
This question is now irrelevant. The “Export as graphic” option is included in the upcoming version 4.0 (which can be tested as a Beta build in http://www.libreoffice.org/index.php/download/pre-releases/)
Currently a Chart can be exported in a number of bitmap formats (PNG, TIF, etc) and even in Windows Metafile format.
An existing bug currently prevents exporting to SVG (https://bugs.freedesktop.org/show_bug.cgi?id=59083)
@Pedro1 I am glad to hear this important feature is finally coming, and really hope the SVG bug would be fixed before release of 4.0 since it would be my preferred format. Thanks!
Now with 4.1, the bug have been fixed. I have added a new answer but I think that could be better if you upgrade your question with this information.
Now that LibreOffice 4.1 have meet the light (2013-11-01), you can use a right click and “Export as graphic” to save the chart as an image in all its formats (inclusive svg, that have been fixed in this release).
Bug 60137 - Allow exporting charts (“Export as Graphic”) also to vector graphics (SVG, PDF,…)
Status: RESOLVED FIXED
You can copy (or cut) and paste your chart into a LibreOffice Draw page, and then, in LibreOffice Draw, select the chart, and export it with File>>Export.
In the Export dialogue box, choose your graphics format and remember to check the ‘selection’ box.
It’s still a bit cumbersome, but at least it’s all done within LibreOffice.
Perhaps a feature request to allow these export options directly from Calc for chart objects?
+1 on having an export option. Preferably do it right, like Gnumeric which allows to save as PNG, JPG, PS, EPS, PDF and better yet: SVG!
@mikebibo Thanks! I do like that I can at least do all of this within LibreOffice. But this feature should really have been implemented by now! Is there a dedicated feature request page for LibreOffice?
For that we have Enhancement Request “Bug 30944 - Allow exporting a single chart (or other object) to different formats (PDF, SVG, PNG, JPG), target document with object size”
Is that a new feature in MS Office 2010? You can also request a new feature for LibreOffice 
Meanwhile the easiest solution is to paste into any image editor/viewer and save in your favorite file format. Personally I use the free IrfanView (it is light and fast) but you can use Gimp 
@Pedro1 I can’t remember how to do it in 2003 (it has been too long!), but in 2007 you should be able to just right click on a chart, and choose the “Save as Image” option.
@penyuan, no you can’t. There is no option to Save as Image. You can Copy as Picture (in Office 2003) but you need to paste it somewhere… That is why I’m asking if it is a new feature in Office 2010
@Pedro1 Sorry my memory is definitely failing me then! Yeah I guess this must be a 2010 feature… sorry about the confusion.
An easy solution from Bug 30944: select the chart, choose File → Export as pdf → selection.
As of November 2014, I am still running LibreOffice 3 on Linux and do not have direct chart graphics export capability, but there is a fairly easy workaround. With Inkscape installed on the computer, I export the chart from Calc to a PDF file, then extract the chart from the PDF using Inkscape. This allows one full freedom to manipulate the graphics in vector format, then export as PNG or save as SVG for use in other applications.
- Click on the chart in LibreOffice Calc to select it; eight small, green squares appear around the edges.
- In the menu bar, click File > Export as PDF…
- In the PDF Options dialog, select Range: “Selection”, then click on the Export button and save the PDF file.
- Start up Inkscape and import the PDF file thus generated. It should contain only the chart, not the spreadsheet, with all graphics in a single group.
- Ungroup everything and delete the page header and footer text, if it is there, plus any objects not needed for your target application.
- Some objects, such as grid lines and tick marks on the axes, may not be visible, because they have no fill or stroke assigned to them, but this can be corrected in Inkscape with the Fill and Stroke toolbox.
Once the fix-ups are done, save the image in SVG format, and if desired, export a copy in PNG format at the desired scale. Using vector graphics allows the chart image to be scaled practically without limit, yet preserving smooth, clear appearance at all scale factors.
Free Inkscape binaries are available for most popular operating systems, including Linux, Mac OS X, and Microsoft Windows.