DAX Progress calculation

OK.

I've played with this a bit. First, the graph must be stepped out of neccessity. When we calculate the progress over days, the formula grabs the progress from the Schedule table and this progress is recorded in discrete chunks and you don't have data for in-between dates. Second, the formula does not work correctly because it's much harder to write correct DAX against a bad model (which this one is). A good model is dimensional, where there are dimensions and a fact table. You only have a fact table. Proper design in Power BI is CRUCIAL to get simple and correct DAX. Third, here's the correct formula in this bad model where the Schedule fact table (as you painted it in your first post) is CONNECTED to the Dates table:

Doc Progress = 
var __lastVisibleDate = MAX( Dates[Date] )
var __selectedDocumentsWithProps =
    CALCULATETABLE(
        SUMMARIZE( 
            Schedule,
            Schedule[Document],
            Schedule[Prop]
        ),
        ALL( Dates )
    )
var __lastDateOfProgress =
    CALCULATE(
        MAX( Schedule[Date] ),
        __selectedDocumentsWithProps,
        ALL( Schedule )
    )
var __documentsWithProgress =
    GENERATEALL(
            __selectedDocumentsWithProps,
        // For each document and the last
        // visible date we have to get the
        // latest row in T. If there are
        // no rows returned, then we'll have
        // to retrieve the proportion for
        // the document anyway.
        topn(1,
            CALCULATETABLE(
                SUMMARIZE( 
                    Schedule,
                    Schedule[Progress],
                    Schedule[Date]
                ),
                Dates[Date] <= __lastVisibleDate
            ),
            Schedule[Date],
            DESC
        )
    )
var __totalProgress =
    if( __lastDateOfProgress >= MIN( Dates[Date] ),
        DIVIDE(
            SUMX(
                __documentsWithProgress,
                Schedule[Prop] * Schedule[Progress]
            ),
            SUMX(
                __documentsWithProgress,
                Schedule[Prop]
            )
        )
    )
RETURN
    __totalProgress

This formula is more complex than necessary only because the model is bad.

By the way, if you want to get a line that's not stepped, you'll have to do a lot more calculations inside the measure. Basically, if you're on the day granularity, you'll have to sense whether the day in the current context is present in Schedule or not and if it's not, you'll have to calculate linear approximation between the nearest days to the one in question that do exist in Schedule. Doable but requires a bit of effort.