Introduce "single value" model for monochrome graph

The text labels for CPU, memory and swap go to the trouble of figuring
out a subset of the values to sum, for a single-value display.

But the monochrome graph simply summed all `curItems` values. This is
less sophisticated than the label behaviour.

Add a `Meter::single_value` member that can be used to output a meter's
single-value sum, and make the graph display use it instead of its own
"all-bars" sum where available.

Implement this in `MemoryMeter`, `SwapMeter` and `CPUMeter`.

This has the effect of making a memory graph not show cached memory, so
will not be permanently at the top when the cache is heavily used, and
the CPU graph will obey the `account_guest_in_cpu_meter` setting. In
both cases, the graph becomes consistent with the text label of the bar.

CPUMeter.c

@@ -73,9 +73,12 @@ static void CPUMeter_updateValues(Meter* this) {
    double percent = Platform_setCPUValues(this, cpu);
    if (isnan(percent)) {
       xSnprintf(this->txtBuffer, sizeof(this->txtBuffer), "offline");
+      this->single_value = 0;
       return;
    }
 
+   this->single_value = percent;
+
    char cpuUsageBuffer[8] = { 0 };
    char cpuFrequencyBuffer[16] = { 0 };
    char cpuTemperatureBuffer[16] = { 0 };

MemoryMeter.c

@@ -41,12 +41,12 @@ static void MemoryMeter_updateValues(Meter* this) {
    this->curItems = MEMORY_METER_AVAILABLE;
 
    /* we actually want to show "used + compressed" */
-   double used = this->values[MEMORY_METER_USED];
+   this->single_value = this->values[MEMORY_METER_USED];
    if (!isnan(this->values[MEMORY_METER_COMPRESSED])) {
-      used += this->values[MEMORY_METER_COMPRESSED];
+      this->single_value += this->values[MEMORY_METER_COMPRESSED];
    }
 
-   written = Meter_humanUnit(buffer, used, size);
+   written = Meter_humanUnit(buffer, this->single_value, size);
    METER_BUFFER_CHECK(buffer, size, written);
 
    METER_BUFFER_APPEND_CHR(buffer, size, '/');

Meter.c

@@ -41,6 +41,7 @@ Meter* Meter_new(const Machine* host, unsigned int param, const MeterClass* type
    this->curItems = type->maxItems;
    this->curAttributes = NULL;
    this->values = type->maxItems ? xCalloc(type->maxItems, sizeof(double)) : NULL;
+   this->single_value = NAN;
    this->total = type->total;
    this->caption = xStrdup(type->caption);
    if (Meter_initFn(this)) {
@@ -330,7 +331,9 @@ static void GraphMeterMode_draw(Meter* this, int x, int y, int w) {
       for (int i = 0; i < nValues - 1; i++)
          data->values[i] = data->values[i + 1];
 
-      if (Meter_comprisedValues(this)) {
+      if (!isnan(this->single_value)) {
+         data->values[nValues - 1] = this->single_value;
+      else if (Meter_comprisedValues(this)) {
          data->values[nValues - 1] = (this->curItems > 0) ? this->values[this->curItems - 1] : 0.0;
       } else {
          double value = 0.0;

Meter.h

@@ -117,6 +117,7 @@ struct Meter_ {
    const int* curAttributes;
    char txtBuffer[METER_TXTBUFFER_LEN];
    double* values;
+   double single_value;
    double total;
    void* meterData;
 };

SwapMeter.c

@@ -33,7 +33,9 @@ static void SwapMeter_updateValues(Meter* this) {
    this->values[SWAP_METER_FRONTSWAP] = NAN;   /* 'frontswap' not present on all platforms */
    Platform_setSwapValues(this);
 
-   written = Meter_humanUnit(buffer, this->values[SWAP_METER_USED], size);
+   this->single_value = this->values[SWAP_METER_USED];
+
+   written = Meter_humanUnit(buffer, this->single_value, size);
    METER_BUFFER_CHECK(buffer, size, written);
 
    METER_BUFFER_APPEND_CHR(buffer, size, '/');