Sparklines don't work

I am a new user of piwik. I want to track my clients sites.
Piwik started to show the stats, but sparklines don’t work.
I talked with my web host (Hostgator) and they said taht they already instaleed php5 with gd.
How can I solve the problem?
Thank you so much!

The screenshot abiut it:
it’s on visitors page.
Thank you!

Right click on the image, click “View image” -> what error do you see?

403 error
I am using All In One WP Security & Firewall and it’s htaccess security.
What shall I change in my htaccess?

Not sure, make it allow CSS files?

Ok, I solved it. Just allowed piwik folder.
Thank you!