How do I plot multiple time series from a Dataset?

The query ds[All,{"Date","Column1"}] returns a list of associations whose values represent date-value pairs. The query ds[All, "Date", "Column1", "Column2"}] returns a list of associations with multiple values for each date. DateListPlot will accept the first format, but not the second.

To plot multiple series, we must reformat the data into a shape that is acceptable to DateListPlot. For example, we could generate multiple lists of date-value pairs:

ds[Transpose /* DateListPlot, {{#Date, #Column1}, {#Date, #Column2}}&]

Discussion

For discussion purposes, we will define a sample dataset:

ds = Range[5] // Query[Dataset
     , <| "date" -> (DatePlus["2005-01-01", {#, "Year"}]&)
        , "v1" -> Sin
        , "v2" -> Cos
        , "v3" -> Sec|>
     ]

As noted in the question, we can plot a single column of values easily:

ds[DateListPlot, {"date", "v1"}]

but the same technique fails for multiple columns:

ds[DateListPlot, {"date", "v1", "v2"}]

The reason is that DateListPlot can handle the date-value pairs that are returned by the first query:

ds[First /* Values, {"date", "v1"}] // Normal
(* <|"date" -> "Sun 1 Jan 2006 00:00:00", "v1" -> Sin[1]|> *)

But DateListPlot does not presently (V11.1) accept an association with more than one value per date as is returned by the second query:

ds[First, {"date", "v1", "v2"}] // Normal
(* <|"date" -> "Sun 1 Jan 2006 00:00:00", "v1" -> Sin[1], "v2" -> Cos[1]|> *)

To plot multiple columns, we must reshape the data to conform to one of the formats acceptable to DateListPlot. For example, it will accept multiple lists of date-values pairs:

ds[Transpose /* DateListPlot, {{#date, #v1}, {#date, #v2}, {#date, #v3}} &]

Alternatively:

ds[(Query[All, {"date", #} /* Values] & /@ {"v1", "v2", "v3"}) /* DateListPlot]

In both queries we had to take care that the date-value pairs were lists -- not associations. This is due to another limitation of DateListPlot. While DateListPlot will accept a single set of associations containing date-value pairs, it cannot presently handle a list of such sets. We must convert the associations to lists to work around this restriction.

 ds = Dataset[{
    <|"Date" -> DateObject[{2016, 6, 28}], "c1" -> 10, "c2" -> 30, "c3" -> 0|>,
    <|"Date" -> DateObject[{2016, 6, 29}], "c1" -> 20, "c2" -> 20, "c3" -> 30|>,
    <|"Date" -> DateObject[{2016, 6, 30}], "c1" -> 30, "c2" -> 10, "c3" -> 0|>}]

DateListPlot[
 Values @ Normal @ ds[All, {"Date", #}] & /@ {"c1", "c2", "c3"},
 PlotMarkers -> Automatic,
 PlotLegends -> {"c1", "c2", "c3"}]

cplot[d_Dataset, c_?VectorQ] :=
 DateListPlot[
  Values @ Normal @ d[All, {"Date", #}] & /@ c,
  PlotMarkers -> Automatic,
  PlotLegends -> c]

cplot[ds, {"c1", "c3"}]