Return a list of n colors, specified by their standard plotly names;
        meant for situations when 1 or more default colors are needed.

        The choice of default colors is hardwired in this function.

        :param n:   Desired number of default colors
        :return:
        

        Plot one or more 2D curves

        :param x:           A Numpy array, with the (common) x-axis values
        :param y:           Either a Numpy array, or a list/tuple of them, with the y-axis values of the curve(s)
        :param title:       [OPTIONAL] Title to use for the overall plot
        :param range_x:     [OPTIONAL] list of the form [t_start, t_end], to initially only show a part of the timeline.
                                Note: it's still possible to zoom out, and see the excluded portion
        :param x_label:     [OPTIONAL] Caption to use for the x-axis
        :param y_label:     [OPTIONAL] Caption to use for the y-axis
        :param curve_labels:[OPTIONAL] String, or list of strings.
                                Label(s) to use for the various curves in the legend and in the hover boxes.
                                If missing, and there's only 1 curve, the legend box won't be shown
        :param legend_title:[OPTIONAL] String to show at the top of the legend box.
                                Ignored if curve_labels wasn't set.
                                If not provided, and the legend box is shown, it will appear as "variable"
        :param colors:      [OPTIONAL] Either a single color (string with standard plotly name, such as "red"),
                                or list of names to use, in order; if None, then use the hardwired defaults
        :param show:        If True, the plot will be shown
                                Note: in JupyterLab, simply returning a plot object (without assigning it to a variable)
                                leads to it being automatically shown

        :return:            A plotly "Figure" object
        

        Using plotly, draw line plots from the values in the given dataframe.

        Note: if this plot is to be later combined with others, use PlotlyHelper.combine_plots()

        :param df:              Pandas dataframe with the data for the plot
        :param x_var:           Name of column with independent variable for the x-axis
        :param fields:          Name, or list of names, of the dataframe columns whose values are to be plotted;
                                    if a list is passed, also display a figure legend;
                                    if None, then display all columns except the one that is declared as the independent variable
        :param colors:          (OPTIONAL) Either a single color (string with standard plotly name, such as "red"),
                                    or list of names to use, in order; if None, then the hardwired default colors are used
        :param title:           (OPTIONAL) Title for the plot
        :param title_prefix:    (OPTIONAL) Strint to prefixed (automatically followed by " 
") to the title
        :param range_x:         (OPTIONAL) list of the form [t_start, t_end], to initially show only a part of the timeline.
                                    Note: it's still possible to zoom out, and see the excluded portion
        :param range_y:         (OPTIONAL) list of the form [y_min, y_max], to initially show only a part of the y values.
                                    Note: it's still possible to zoom out, and see the excluded portion
        :param x_label:         (OPTIONAL) Caption to use for the x-axis
        :param y_label:         (OPTIONAL) Caption to use for the y-axis.  Default: "Y"
        :param legend_header:   (OPTIONAL) Caption to use at the top of the legend box.
                                            Only applicable if more than 1 curve is being shown.
        :param vertical_lines_to_add:  (OPTIONAL) Ignored if the argument `show_intervals` is specified.
                                    Value, or list, or tuple, or Numpy array, or Pandas series,
                                    of x-coordinate(s) at which to draw thin vertical dotted gray lines.
                                    If the number of vertical line is so large as to overwhelm the plot,
                                    only a sample of them is shown.
                                    Note that vertical lines, if requested, go into the plot's "layout";
                                    as a result they might not appear if this plot is later combined with another one.
        :param show_intervals:  (OPTIONAL) If True, it over-rides any value passed to the `vertical_lines` argument,
                                    and draws thin vertical dotted gray lines at all the x-coords
                                    of the data points in the saved history data;
                                    also, it adds a comment to the title.
        :param show:            If True, the plot will be shown
                                    Note: on JupyterLab, simply returning a plot object (without assigning it to a variable)
                                          leads to it being automatically shown

        :return:                A plotly "Figure" object
        

        Combine together several existing plotly plots into a single one (with combined axes)

        EXAMPLE:
                    from life123 import PlotlyHelper
                    p1 = PlotlyHelper.plot_pandas(various args, show=False)
                    p2 = PlotlyHelper.plot_pandas(various args, show=False)
                    PlotlyHelper.combine_plots([p1, p2], other optional args)

        :param fig_list:    List or tuple of plotly "Figure" objects (as returned by several functions)
        :param title:       [OPTIONAL] The title to use for the overall plot
        :param x_label:     [OPTIONAL] Caption to use for the x-axis; if not specified, use that of the 1st plot
        :param y_label:     [OPTIONAL] Caption to use for the y-axis; if not specified, use that of the 1st plot
        :param xrange:      [OPTIONAL] list of the form [t_start, t_end], to initially only show a part of the timeline.
                                Note: it's still possible to zoom out, and see the excluded portion
        :param legend_title:[OPTIONAL] String to show at the top of the legend box
        :param curve_labels:[OPTIONAL] List of labels to use for the various curves in the legend
                                and in the hover boxes; if not specified, use the titles of the individual plots
        :param show:        If True, the plot will be shown
                                Note: on JupyterLab, simply returning a plot object (without assigning it to a variable)
                                      leads to it being automatically shown
        :return:            A plotly "Figure" object for the combined plot
        

        Combine into a vertical grid 2 plotly graph objects (now treated as subplots)

        :param fig1:
        :param fig2:
        :param title1:
        :param title2:
        :param title_combined:
        :param height           [OPTIONAL] The overall height of the combined plots
        :return:                A combined plot (a plotly "figure" object)