Source: Team_Talk
Hornets complete striker deal
The 23-year-old was out of contract at Brunton Park, but negotiations are continuing between the clubs over a compensation fee.
Graham, who began his career with Middlesbrough and has spent time on loan at Derby, Blackpool and Leeds, has agreed a two-year deal at Vicarage Road with the option of a further 12 months.
He becomes Mackay's second signing of the summer following the arrival of Scott Severin from Aberdeen.