You are trying to enable a series before adding it to chart
Move the line
chtCampaignHits.Series[s].Enabled = true;
next to
chtCampaignHits.Series.Add(new Series(s));
Your code should be
chtCampaignHits.Series.Add(new Series(s));
chtCampaignHits.Series[s].Enabled = true;
chtCampaignHits.Series[s].Label = s;
Updated answer
Your code should be
chtCampaignHits.Series.Add(s);
chtCampaignHits.Series[s].Enabled = true;
chtCampaignHits.Series[s].Label = s;