Allow deselect on dropdowns

[MikeLister]

Bar with dropdown e.g. from line 26:

// Add the placeholder option
optns.append('option'); // Placeholder text

optns
	.selectAll('option.option')
	.data(uniqueOptions)
	.enter()
	.append('option')
	.attr('value', (d) => d)
	.text((d) => d);



		//add some more accessibility stuff
d3.select('input.chosen-search-input').attr('id', 'chosensearchinput');
d3.select('div.chosen-search')
	.insert('label', 'input.chosen-search-input')
	.attr('class', 'visuallyhidden')
	.attr('for', 'chosensearchinput')
	.html('Type to select an area');

$('#optionsSelect').chosen({placeholder_text_single: 'Select an option', allow_single_deselect: true})