Hi Erdem,
Before I have upgraded DS1.1 to SP 3 I had the same issue! But only with Internet Explorer as standard browser. In a test szenario with SP1 I have changed the browser to IE to Firefox and the legend labels was visible.
After I had upgraded Design Studio 1.1 from SP 1 to SP 3 the Issue was completely fixed. With SP 3 the chart legends are visible regardless of which browser (IE, Firefox, Chrome) you are use.
Please download SP3 and upgrade your Design Studio client.
Hope it helps!
Best regards,
Michael