Highcharts editor plot lines configuration crash the editor

[berdeter]

Message : type error (Intermediate value).concat is not a function.

Several problems encountered :

• plot lines is documented as an array and the editor shows a single entry similar to #89
• whenever you try to enter a value in this dialog you get a toast with the error
• as soon as you have had the error your whole configuration is doomed and all is broken untill you reload the page. Removing the value is not enough

[cvasseng]

See #95