disable default selection of button in range selector buttons

Hello all,

Right now, enabling the use of buttons in layout.xaxis.rangeselector makes it such that the button 'all' is always selected by default, no matter where it is positioned in the list of buttons. If the button 'all' is not present, the display is still rendered as if it was selected by default.

May we add a default (boolean) property inside a button to know which should be selected by default when first displaying the graph ? This could help to focus on a key point of the data, while allowing the user to zoom out by clicking the 'all' button or by double-clicking on the graph.
This property would be set to false by default, and if no buttons show a true value for this property, then the usual 'all' display shall be rendered.

Example :
Usual display

var layout = {
xaxis: {
            autorange: true,
            rangeselector: {
                buttons: [
                    {
                        count: 1,
                        label: '1m',
                        step: 'month',
                        stepmode: 'backward'
                    },
                    {
                        count: 6,
                        label: '6m',
                        step: 'month',
                        stepmode: 'backward'
                    },
                    {   
                        step: 'all'
                    }
                ]
            },
            rangeslider: {autorange: true},
            type: 'date'
        }
};

In the second case, when loading the page, we already have the 6 months zoom.
6 months button selected by default

var layout = {
        xaxis: {
            autorange: true,
            rangeselector: {
                buttons: [
                    {
                        count: 1,
                        label: '1m',
                        step: 'month',
                        stepmode: 'backward'
                    },
                    {
                        count: 6,
                        label: '6m',
                        step: 'month',
                        stepmode: 'backward',
                        default: true
                    },
                    {   
                        step: 'all'
                    }
                ]
            },
            rangeslider: {autorange: true},
            type: 'date'
        }
};

Inizia dalla gestione dei pulsanti xaxis.rangeselector descritta nell’issue e traccia il modo in cui viene scelto l’intervallo selezionato inizialmente. Il lavoro è completato quando un pulsante con default: true controlla l’intervallo iniziale, mentre il comportamento esistente di “all” rimane il fallback quando nessun pulsante è contrassegnato come predefinito.