diff --git a/lib/tricle/email_helper.rb b/lib/tricle/email_helper.rb
index <HASH>..<HASH> 100644
--- a/lib/tricle/email_helper.rb
+++ b/lib/tricle/email_helper.rb
@@ -90,7 +90,7 @@ module Tricle
 
     def sparkline(metric)
       values = metric.weekly_values(13)
-      attachment_url = "http://sparklines.herokuapp.com/api/v1.png?values=#{values.join(',')}"
+      attachment_url = "https://sparklines.herokuapp.com/api/v1.png?values=#{values.join(',')}"
       image_tag(attachment_url, alt: 'sparkline').html_safe
     end
   end
use htttps url for sparkle